Imaging medium

ABSTRACT

Acid can be generated by exposing a mixture of a superacid precursor and a dye to actinic radiation of a first wavelength which does not, in the absence of the dye, cause decomposition of the superacid precursor to form the corresponding superacid, thereby causing absorption of the actinic radiation and decomposition of part of the superacid precursor, with formation of a protonated product derived from the dye, then irradiating the mixture with actinic radiation of a second wavelength, thereby causing decomposition of part of the remaining superacid precursor, with formation of unbuffered superacid. Preferably, following these irradiations, the imaging medium is heated while the superacid is admixed with a secondary acid generator capable of being thermally decomposed to form a second acid, the thermal decomposition of the secondary acid generator being catalyzed by the presence of the superacid. The acid generation process may be used for imaging by bringing the superacid or second acid into contact with an acid-sensitive material which changes color on contact with acid, or the superacid may be used to trigger polymerization, depolymerization or other reactions.

REFERENCE TO PARENT APPLICATION

This application is a continuation-in-part of application Ser. No.07/965,161, filed Oct. 23, 1992, now U.S. Pat. No. 5,286,612.

REFERENCE TO RELATED APPLICATIONS AND PATENTS

Attention is directed to copending application Ser. No. 07/965,172, andits divisional application Ser. No. 08/106,353, filed Aug. 13, 1993;these two applications describe and claim a process and imaging mediumgenerally similar to those of the present invention, but in which thebreakdown of a squaric acid derivative is initiated thermally.

Attention is also directed to copending application Ser. No. 07/965,162(now U.S. Pat. No. 5,286,612) and its continuation-in-part, applicationSer. No. 08/141,866 now U.S. Pat. No. 5,346,736, of even date herewithand assigned to the same assignee as the present application; these twoapplications describe and claim a process and imaging medium generallysimilar to those of the present invention but in which a superacidprecursor is exposed to actinic radiation effective to generatesuperacid from the superacid precursor, and the resultant superacid isheated while admixed with a secondary acid generator (typically asquaric acid derivative or an oxalate) capable of thermally decomposingto produce an acid, thereby causing production of a secondary acid.

Finally, attention is directed to copending application Ser. No.08/084,759, filed Sep. 17, 1993 and assigned to the same assignee as thepresent application; this application describes and claims a process andimaging medium using a mixture of a diaryl iodonium salt and asquarylium dye capable of absorbing infra-red radiation having awavelength within the range of about 700 to about 1200 nm, the dyehaving a squarylium ring the 1- and 3-positions of which are eachconnected, via a single Sp² carbon atom, to a pyrylium, thiopyrylium,benzpyrylium or benzthiopyrylium moiety, at least one of the Sp² carbonatoms having a hydrogen atom attached thereto, and the 2-position of thesquarylium ring bearing an O⁻, amino or substituted amino, orsulfonamido group. The mixture is irradiated with infra-red radiationhaving a wavelength within the range of about 700 to about 1200 nm,thereby causing absorption of the radiation by the squarylium dye andformation of acid in the mixture.

BACKGROUND OF THE INVENTION

This invention relates to a process for generation of acid and forimaging, and to an imaging medium for use in this imaging process.

Some conventional non-silver halide photosensitive compositions, forexample photoresists, contain molecules which are inherentlyphotosensitive, so that absorption of a single photon brings aboutdecomposition of only the single molecule which absorbs the photon.However, a dramatic increase in the sensitivity of such photosensitivecompositions can be achieved if the photosensitive molecule initiates asecondary reaction which is not radiation-dependent and which effectsconversion of a plurality of molecules for each photon absorbed. Forexample, photoresist systems are known in which the primaryphotochemical reaction produces an acid, and this acid is employed toeliminate acid-labile groups in a secondary, radiation-independentreaction. See, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706;Reichmanis et al., Chemical Amplification Mechanism forMicrolithography, Chem. Mater.,3(3), 394 (1991) and Berry et al.,Chemically Amplified Resists for I-line and G-line Applications, SPIE,1262, 575 (1990). Also, U.S. Pat. No. 5,084,371 describes aradiation-sensitive mixture which contains a water-insoluble binderwhich comprises a mixture of phenolic and novolak polymers and which issoluble or dispersible in aqueous alkali, and an organic compound whosesolubility in alkaline developer is increased by acid, and which alsocontains at least one acid-cleavable group, and in addition a furthergroup which produces a strong acid upon exposure to radiation.

U.S. Pat. No. 4,916,046 describes a positive radiation-sensitive mixtureusing a monomeric silylenol ether, and a recording medium producedtherefrom. This patent also contains an extensive discussion ofradiation-sensitive compositions which form or eliminate an acid onirradiation. According to this patent, such radiation-sensitivecompositions include diazonium, phosphonium, sulfonium and iodoniumsalts, generally employed in the form of their organic solvent-solublesalts, usually as deposition products with complex acids such astetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonicacid and hexafluoroarsenic acid; halogen compounds, in particulartriazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyroneswhich contain trichloromethyl or tribromomethyl groups; aromaticcompounds which contain ring-bound halogen, preferably bromine; acombination of a thiazole with 2-benzoylmethylenenaphthol; a mixture ofa trihalomethyl compound with N-phenylacridone; α-halocarboxamides; andtribromomethyl phenyl sulfones.

The aforementioned phosphonium, sulfonium and iodonium salts aresuperacid precursors which, upon exposure to ultraviolet radiation,decompose to produce superacids, that is to say acids with a pK_(a) lessthan about 0. Other materials decompose to produce superacids in asimilar manner. However, all the superacid precursors requireultraviolet to blue visible radiation for decomposition (see, forexample, Crivello and Lam, Dye-Sensitized Photoinitiated CationicPolymerization, J. Polymer Sci., 16, 2441 (1978)), and the need for thisradiation is disadvantageous when it is desired to produce highresolution images, which are most conveniently produced by laserimaging. In the present state of technology, diode lasers emitting atnear infra-red wavelengths of about 700 to 1200 nm. provide the highestoutput per unit cost. Near infra-red solid state lasers emitting atabout 1000-1200 nm. are also useful in imaging processes, whileultraviolet lasers are costly. Accordingly, it is desirable to find someway in which superacid precursors can be rendered susceptible toinfra-red radiation in order that imaging of a superacidprecursor-containing medium can be effected using an infra-red laser.

It is already known that various sensitizing dyes can catalyze thedecomposition of superacid precursors upon exposure to wavelengths towhich the superacid precursors are not sensitive in the absence of thesensitizing dye. Unfortunately, due to the difficulty of protonating thesuperacid anion consequent upon the very low pK_(a) of the superacid,the sensitizing dye is protonated by the superacid, so that nounbuffered superacid is produced in the medium (i.e., the sensitizingdye buffers the superacid produced). Since no unbuffered superacid isreleased into the medium, these processes cannot be used to trigger anysecondary reaction which requires the presence of unbuffered strongacid, such as the reactions used in many photoresists, as described inthe aforementioned patents.

(The term "unbuffered superacid" is used herein to refer to superacidwhich is not buffered by the sensitizing dye, and which thus provides anacidic species stronger than that provided by buffered superacid, thatis to say superacid buffered by the sensitizing dye. Because of theextreme acidity of superacids and their consequent tendency to protonateeven species which are not normally regarded as basic, it is possible,and indeed likely, that "unbuffered superacid" will in fact be presentas a species buffered by some component of the imaging medium less basicthan the sensitizing dye. However, such buffering by other species maybe ignored for present purposes, so long as superacid is present as anacidic species stronger than that provided by superacid buffered by thesensitizing dye.)

This invention provides a process for generation of acid which enables amedium containing a superacid precursor and a sensitizing dye, which ismore easily protonated than the superacid anion, to be imaged withradiation of a frequency to which the superacid precursor is notsensitive, so as to produce unbuffered superacid in the medium. Byincluding an acid-sensitive material in the medium, the process can beused for imaging.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a process for generation of acid,which process comprises:

providing a medium containing a mixture of a superacid precursor and adye capable of absorbing actinic radiation of a first wavelength whichdoes not, in the absence of the dye, cause decomposition of thesuperacid precursor to form the corresponding superacid, the superacidprecursor being capable of being decomposed by actinic radiation of asecond wavelength shorter than the first wavelength;

irradiating the medium with actinic radiation of the first wavelength,thereby causing absorption of the actinic radiation, and decompositionof part of the superacid precursor, without formation of unbufferedsuperacid but with formation of a protonated product derived from thedye; and

thereafter irradiating the medium with actinic radiation of the secondwavelength, thereby causing decomposition of part of the remainingsuperacid precursor, with formation of unbuffered superacid.

In a preferred form of this process, only part of the medium isirradiated with the actinic radiation of the first wavelength, but alarger portion of the medium is irradiated with the actinic radiation ofthe second wavelength, such that unbuffered superacid is generated inthe part of the medium exposed to the radiation of both the first andsecond wavelengths, but no unbuffered superacid is generated in the partof the medium exposed to the radiation of the second wavelength but notto the radiation of the first wavelength. Desirably, the medium isimagewise exposed to the actinic radiation of the first wavelength sothat the unbuffered superacid generated forms an image.

This invention also provides an imaging medium comprising:

a superacid precursor; and

a dye capable of absorbing actinic radiation of a first wavelength,

the superacid precursor being decomposed to form a superacid by actinicradiation of a second wavelength shorter than the first wavelength, butnot being decomposed by actinic radiation of the first wavelength in theabsence of the dye, the superacid produced by decomposition of thesuperacid precursor being capable of forming a protonated productderived from the dye; and

a secondary acid generator capable of being thermally decomposed to forma second acid, the thermal decomposition of the secondary acid generatorbeing catalyzed in the presence of the superacid.

Finally, this invention provides an imaging medium comprising:

a superacid precursor and an infra-red dye capable of absorbing infraredradiation having a wavelength within the range of about 700 to about1200 nm, the superacid precursor being capable of being decomposed byultraviolet radiation having a wavelength in the range of about 180 toabout 400 nm to form a superacid, the superacid precursor not beingdecomposed by infra-red radiation having a wavelength within the rangeof about 700 to about 1200 nm in the absence of the infra-red dye.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 A-1D show the acid concentrations in the exposed and unexposedregions of the imaging medium during the various steps of a preferredprocess of the present invention;

FIG. 2 shows a synthesis of a squaric acid derivative of Formula Ibelow; and

FIG. 3 is a schematic cross-section through an imaging medium of thepresent invention as it is being passed between a pair of hot rollersduring the imaging process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As already mentioned, the present process employs a medium containing amixture of a superacid precursor and a dye. The dye (which mayhereinafter be referred to as the "sensitizing dye") is capable ofabsorbing actinic radiation of a first wavelength which does not, in theabsence of the sensitizing dye, cause decomposition of the superacidprecursor to form the corresponding superacid; typically this firstwavelength is in the range of about 700 to about 1200 nm, so that thesensitizing dye is a near infra-red dye. Also, the superacid precursoris capable of being decomposed by actinic radiation of a secondwavelength shorter than the first wavelength; typically this secondwavelength is in the range of about 400 to about 180 nm, so that theactinic radiation of the second wavelength can be conveniently suppliedby ultraviolet sources (e.g., a mercury arc lamp) which are readilyavailable and will be familiar to those skilled in the art.

As is well known to those familiar with superacid precursors, superacidprecursors may require the presence of a precursor sensitizer, typicallya polycyclic hydrocarbon such as pyrene, to enable the superacidprecursor to break down upon irradiation with ultraviolet or otheractinic radiation, thereby producing superacid. Accordingly, referencesherein to a superacid precursor shall be construed to refer to a mixtureof superacid precursor and precursor sensitizer, if the superacidprecursor is one which requires the presence of such a precursorsensitizer.

The medium is first irradiated with actinic radiation of the firstwavelength, thereby causing absorption of the actinic radiation anddecomposition of part of the superacid precursor, with formation of aprotonated product derived from the dye. Thereafter, the medium isirradiated with actinic radiation of the second wavelength, therebycausing decomposition of part of the remaining superacid precursor, withformation of unbuffered superacid.

Since the purpose of the present invention is to produce unbufferedsuperacid, which may be used for various purposes as discussed below, itis highly desirable that the process be conducted under essentiallyanhydrous conditions; as chemists are well aware, the most powerfulacidic species which can exist in the presence of more than oneequivalent of water is the hydroxonium (hydronium) ion, [H₃ O]⁺.Accordingly, if the medium in which the present process is conductedcontains water, at least part of the superacid produced by the presentprocess will simply generate hydroxonium ion. However, in the absence ofwater, the superacid yields an acidic species much stronger thanhydroxonium ion, and this acidic species can be used for purposes forwhich hydroxonium ion cannot, for example the acid-catalyzeddecomposition of various secondary acid generators, as discussed indetail below. Typically, the present process is carried out with thesuperacid precursor and the dye dispersed in a polymeric binder, andsuch binders can readily be chosen to provide an essentially anhydrousenvironment for the process.

For the present process to occur, it is necessary that the sensitizingdye, having absorbed the radiation of the first wavelength, be capableof initiating the decomposition of the superacid precursor. As iswell-known to those skilled in the art, for such initiation to occur, itis necessary to choose the sensitizing dye and the superacid precursorso that the excited state of the sensitizing dye is capable of reducingthe superacid precursor. The choice of appropriate pairs of sensitizingdyes and superacid precursors may be made empirically, althoughtechniques familiar to those skilled in the art, such as use of theRehm-Weller Equation, may be used to reduce the amount of empiricaltesting necessary.

As already noted, in a preferred form of the process, only part of themedium is irradiated with the actinic radiation of the first wavelengthbut a larger portion of the medium is irradiated with the actinicradiation of the second wavelength, such that unbuffered superacid isgenerated in the part of the medium exposed to the radiation of both thefirst and second wavelengths, but no unbuffered superacid is generatedin the part of the medium exposed to the radiation of the secondwavelength but not to the radiation of the first wavelength. Desirably,the medium is imagewise exposed to the actinic radiation of the firstwavelength so that the unbuffered superacid generated in the exposedareas of the medium forms a latent "image" in acid; this image is notnecessarily visible to the unaided human eye but may be converted to avisible or otherwise useful image (e.g., a printing plate) as describedbelow.

The superacid produced by the present process may be used to carry outany of the reactions which have hitherto been carried out usingsuperacid generated by prior art processes. For example, the imagingmedium may comprise a monomer or oligomer which polymerizes in thepresence of the unbuffered superacid. If such a medium is imagewiseexposed by the present process, in the part of the medium exposed to theradiation of both the first and second wavelengths, the monomer oroligomer polymerizes, but in the part of the medium not exposed to theradiation of the first wavelength, the monomer remains substantiallyunpolymerized. Alternatively, the imaging medium may comprise a polymerwhich depolymerizes in the presence of the unbuffered superacid. Whensuch a medium is imagewise exposed by the present process, in the partof the medium exposed to the radiation of both the first and secondwavelengths, the polymer depolymerizes, but in the part of the mediumnot exposed to the radiation of the first wavelength, the polymerremains substantially polymerized. The imaging medium may also comprisea polymer the solubility of which in a solvent changes in the presenceof unbuffered superacid. Following exposure of the medium to theradiation of both the first and second wavelengths, the medium istreated with the solvent, whereby the polymer is removed from one of theexposed and unexposed areas of the medium (i.e., the areas of the mediumexposed and not exposed respectively to the radiation of the firstwavelength), but is not removed from the other of these areas. Thus, anyof these types of imaging medium can act as a photoresist.

A further form of the present imaging medium comprises a polymer theadhesion of which to a material changes in the presence of theunbuffered superacid. Following exposure of the medium to the radiationof both the first and second wavelengths, the polymer is contacted withthis material, so that either the exposed or the unexposed areas of themedium adheres to the material, while the other of these areas does notadhere. For example, the present imaging medium may comprise a substratein contact with one face of the layer(s) containing the imagingcomponents (i.e., the superacid precursor, sensitizing dye and polymer),and a topcoat on the opposed side of the layer(s) containing the imagingcomponents. The polymer is chosen (for example) so that before exposureto unbuffered superacid it adheres more strongly, whereas after exposureto unbuffered superacid it adheres less strongly to the substrate thanto the topcoat. After imagewise exposure of the medium to the radiationof the first and second wavelengths and, optionally, heating, thesubstrate and topcoat are peeled away from one another. In unexposedareas, the polymer remains more adherent to the substrate than to thetopcoat, and remains with the substrate, whereas in exposed areas, thepolymer adheres less strongly to the substrate than to the topcoat, andconsequently remains with the topcoat. Thus, upon peeling, thepolymer-containing layer will fracture, with the unexposed partsremaining on the substrate and the exposed parts being removed with thetopcoat.

Alternatively, the material with which the polymer is brought intocontact after exposure can be a pulverulent material, for example atoning powder. An imaging medium of this type may comprise a polymerwhich is essentially non-tacky prior to exposure but which becomes tackyafter exposure. After exposure, the toning powder is spread over theimaging medium, and adheres only to the exposed areas of polymer. Excesstoning powder may then be removed, for example by blowing air across theimaging medium, thus leaving a visible image formed by the toning powderadhering only to the exposed areas of the imaging medium.

In another type of imaging medium of the present invention, the quantityof acid generated in the medium by the present process is increased("amplified") by heating the medium, following the irradiation with theactinic radiation of the second wavelength, while the superacid isadmixed with a secondary acid generator capable of superacid-catalyzeddecomposition to form a second acid, the thermal decomposition of theacid generator being catalyzed by the presence of the superacid. Whensuch an imaging medium is imagewise exposed to actinic radiation of thefirst wavelength, in the part of the medium irradiated with the actinicradiation of the first wavelength, the superacid catalyzes thedecomposition of the secondary acid generator and the second acid isformed, whereas the part of the medium not irradiated with the actinicradiation of the first wavelength remains essentially free from thesecond acid.

The chemical changes which occur in exposed and unexposed regions of apreferred imaging medium of the present invention are shown in Table 1below, while the corresponding changes in acid concentration in exposedand unexposed areas are shown in FIGS. 1A-1D.

                  TABLE 1                                                         ______________________________________                                        EXPOSED AREA       UNEXPOSED AREA                                             Component      Moles   Component     Moles                                    ______________________________________                                        PRIOR TO EXPOSURE                                                             [DYE]          1       [DYE]         1                                        Secondary acid generator                                                                     10      Secondary acid                                                                              10                                       Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                              5       generator                                                                     Ph.sub.2 I.sup.+ PF.sub.6 .sup.-                                                            5                                        AFTER IMAGEWISE INFRA-RED EXPOSURE                                            Ph-[DYE-H].sup.+ PhIPF.sub.6.sup.-                                                           0.75    [DYE]         1                                        [DYE]          0.25    Secondary acid                                                                              10                                       Secondary acid generator                                                                     10      generator                                              Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                              4.25    Ph.sub.2 I.sup.+ PF.sub.6 .sup.-                                                            5                                        AFTER BLANKET ULTRA-VIOLET EXPOSURE                                           Ph-[DYE-H].sup.+ PhIPF.sub.6.sup.-                                                           0.75    [DYE]         0.25                                     [DYE-H].sup.+ PF.sub.6.sup.-                                                                 0.25    [DYE-H].sup.+ PF.sub.6.sup.-                                                                0.75                                     HPF.sub.6      0.5     Secondary acid                                                                              10                                       Secondary acid generator                                                                     10      generator                                              Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                              3.5     Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                             4.25                                     AFTER HEATING                                                                 Ph-[DYE-H].sup.+ PhIPF.sub.6.sup.-                                                           0.75    [DYE]         0.25                                     [DYE-H].sup.+ PF.sub.6.sup.-                                                                 0.25    [DYE-H].sup.+ PF.sub.6.sup.-                                                                0.75                                     HPF.sub.6      0.5     Secondary acid                                                                              10                                       Second acid    10      generator                                              Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                              3.5     Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                             4.25                                     AFTER BASE ADDITION                                                           Ph-[DYE-H].sup.+ PhIPF.sub.6.sup.-                                                           0.75    [DYE]         1                                        [DYE-H].sup.+ PF.sub.6.sup.-                                                                 0.25    [BASE-H].sup.+ PF.sub.6.sup.-                                                               0.75                                     [BASE-H].sup.+ PF.sub.6.sup.-                                                                0.5     Secondary acid                                                                              10                                       Second acid    9       generator                                              Base/second acid salt                                                                        1       Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                             4.25                                     Ph.sub.2 I.sup.+ PF.sub.6.sup.-                                                              3.5                                                            ______________________________________                                    

As shown in Table 1, prior to exposure both the exposed and unexposedregions comprise a quantity (shown in Table 1 as 1 mole for simplicity;all references to moles concerning Table 1 refer to moles per unit areaof the imaging medium) of an infra-red sensitizing dye, a larger molarquantity of a superacid precursor (5 moles of Ph₂ I³⁰ PF₆ ⁻ are shown inTable 1; a suitable quantity of a precursor sensitizer, such as pyrene,is also included in the medium but is not shown in Table 1) and a stilllarger molar quantity (10 moles are shown in Table 1) of a secondaryacid generator.

The imaging medium is first imagewise irradiated with infra-redradiation of a frequency absorbed by the sensitizing dye, the amount ofradiation applied being sufficient to cause the infra-red dye to bringabout decomposition of less than one mole (0.75 mole is used forillustration in Table 1 and FIG. 1) of the superacid precursor. In thearea of the imaging medium exposed to the infra-red radiation(hereinafter referred to as the "exposed area"), upon absorbing theinfra-red radiation, the sensitizing dye transfers an electron to thesuperacid precursor, which then fragments to produce a phenyl radicaland phenyl iodide. Although the secondary reactions which follow thisfragmentation of the superacid precursor are not entirely understood atpresent, one pathway for further reaction may be combination of aradical cation derived from the sensitizing dye with the phenyl radicalderived from the superacid precursor, and subsequent loss of a protonfrom the sensitizing dye to form a protonated species derived from thesensitizing dye and designated "Ph-[DYE-H]³⁰ " in Table 1, with chargebalancing being effected by an anion derived from the superacidprecursor. Each sensitizing dye molecule transfers only a singleelectron, and hence generates a single proton, before being converted tothe protonated species, and this protonated species does not carry outthe electron transfer reaction. Hence, since each sensitizing dyemolecule brings about breakdown of only a single molecule of superacidprecursor before being deactivated, the sensitizing dye will not bringabout decomposition of a greater molar quantity of the superacidprecursor than the molar quantity of sensitizing dye originally present,and the superacid generated is completely buffered by the dye. Hence,following the infra-red exposure, no unbuffered superacid is present inthe exposed area. At this stage, the secondary acid generator in theexposed area remains unchanged. In the unexposed area, the infra-redirradiation effects no change in any of the components of the imagingmedium.

The acid concentrations in the exposed and unexposed regions followingthis first step of the process are shown schematically in FIG. 1A, inwhich acid concentration is plotted along a line across the medium inwhich section BC is in an exposed area, while sections AB and CD are inunexposed areas. As shown in FIG. 1A, following the first step of theprocess, no acid is present in unexposed areas AB and CD, while thelevel of acid present in exposed area BC is below a threshold level T,which represents the level of acid which can be buffered by thesensitizing dye; theoretically, the level of acid in area BC should be0.75 T. Hence, as already stated, all of the acid present in exposedarea BC is buffered by the sensitizing dye, and the imaging mediumcontains 0.75 mole of the Ph-[DYE-H]⁺ and no unbuffered superacid.

In the next stage of the imaging process, the whole of the imagingmedium is irradiated with ultraviolet radiation effective to causebreakdown of the superacid precursor, with generation of unbufferedsuperacid. The amount of ultraviolet radiation irradiated is chosen soas to produce a molar amount of superacid less than the molar amount ofdye present in the unexposed medium, and in Table 1 is shown assufficient to produce 0.75 mole of superacid. As illustrated in Table 1and FIG. 1B, in the exposed area BC of the imaging medium (i.e., in thearea exposed to the infra-red radiation), the additional 0.75 mole ofsuperacid generated by the ultra-violet exposure, combined with the 0.75mole generated by the infra-red exposure, exceeds the threshold level T,and thus protonates all the sensitizing dye present and leavesadditional superacid in unbuffered form. (For purposes of illustration,FIG. 1B shows the acid generated in the infra-red and ultra-violetexposures separately, although of course no difference existschemically.) In the unexposed areas AB and CD, on the other hand, onlythe 0.75 mole of superacid generated by the ultra-violet exposure ispresent, the acid concentration remains below the threshold level T, andall of the superacid produced is buffered by the sensitizing dye, sothat no unbuffered superacid is present following the ultravioletirradiation. (As shown in Table 1, the buffered complex formed by thesensitizing dye and the superacid precursor in the unexposed areas ABand CD during the ultraviolet irradiation differs from that produced inthe exposed area BC during the infra-red irradiation. During theultraviolet irradiation, the superacid precursor typically transfers aphenyl group not to the dye but rather to the precursor sensitizer(which is effectively non-basic), so that only a proton comes to resideon the infra-red sensitizing dye. However, this difference between thetwo buffered complexes does not affect the present process, since bothcomplexes efficiently buffer the superacid.)

Thus, at the end of the blanket ultraviolet irradiation, unbufferedsuperacid is present in the exposed area, whereas in the unexposed areano unbuffered superacid is present, all the superacid generated beingbuffered by the sensitizing dye.

The two steps already described may be the only steps of the presentprocess. If, for example, the present process is to be used to bringabout polymerization of a monomer or oligomer, or depolymerization of apolymer, the unbuffered superacid produced in area BC in FIG. 1B may beused directly to carry out the desired polymerization ordepolymerization. It will be appreciated that, in such polymerization ordepolymerization processes, the secondary acid generator can be omittedfrom the imaging medium.

However, in a preferred process of the invention, the imaging medium isnext heated. In the exposed area BC, the unbuffered superacid presentcatalyzes the decomposition of the secondary acid generator, therebyproducing a large quantity of the second acid (10 moles are shown by wayof example in Table 1; FIG. 1C is not strictly to scale). However, inthe unexposed areas AB and CD, no unbuffered superacid is present, andthe dye-superacid complex does not catalyze the decomposition of thesecondary acid generator, so that essentially no decomposition of thesecondary acid generator occurs and essentially no second acid isgenerated.

In the final step of the preferred process, as discussed in more detailbelow, a quantity of base is introduced into the imaging medium; 1.5moles of base are shown in Table 1. The effect of this addition of baseis to reduce acid levels throughout the imaging medium, as indicated bythe box U in FIG. 1D. The addition of this base serves to ensure that,if a small amount of uncatalyzed thermal decomposition of the secondaryacid generator does occur in unexposed areas AB and CD during theheating step, the small amount of second acid resulting will beneutralized by base before the second acid can effect changes in anacid-sensitive material, as described in more detail below. Although theaddition of the base does reduce the amount of free acid present in theexposed area BC, this reduction is not significant since a more thanadequate amount of second acid remains in the exposed area BC to affectan acid-sensitive material. The addition of base to the unexposed areasAB and CD leaves a surplus of base in these areas and thus serves toensure that, if minor decomposition of the superacid precursor doesoccur after the present process has been completed, the minor amounts ofsuperacid generated will be neutralized by the base and thus will notaffect acid-sensitive material present in these unexposed areas.

From the foregoing description, it will be seen that, in the exposedarea, the superacid catalyzes the breakdown of the secondary acidgenerator, so that the final quantity of second acid present issubstantially larger than the quantity of superacid produced directly bythe actinic radiation acting on the superacid precursor, although ofcourse the secondary acid is typically a weaker acid than the superaciditself. This "chemical amplification" of the superacid by the secondaryacid generator increases the number of moles of acid generated pereinstein of radiation absorbed, and thus increases the contrast of theimage produced by the present process as compared with simple generationof superacid by a superacid precursor. In practice, it has been foundthat, under proper conditions, at least 20 moles of second acid can beliberated for each mole of unbuffered superacid present in the exposedareas following the ultra-violet irradiation.

One of the advantages of the present process is that, at least in manypreferred embodiments of the invention, it is possible to compensate forany premature breakdown of the superacid precursor which may occur priorto use of the imaging medium, for example as a result of exposure of theimaging medium to ambient infra-red or ultra-violet radiation duringtransportation and storage or because the combination of the superacidprecursor and the sensitizing dye undergoes slow decomposition onprotracted storage. With most infra-red sensitizing dyes, the protonatedproducts derived from the sensitizing dye will absorb at a wavelengthsignificantly different from the unprotonated sensitizing dye, so thatit will be possible to differentiate between the unprotonated dye andprotonated product by measuring absorption at an appropriate infra-redwavelength. The amount of infra-red and ultra-violet irradiation can beadjusted to ensure that the present process works properly even if somedecomposition of the superacid precursor has taken place prior to use ofthe medium.

For example, to take an extreme case purely for purposes ofillustration, suppose that the imaging medium shown in Table 1 isexposed to so much infra-red radiation during storage and transit thathalf of the infra-red sensitizing dye has already been converted to thePh-[DYE-H]⁺ form prior to use, with corresponding breakdown of 0.5 moleof superacid precursor, so that in all areas the medium initiallycontains 0.5 mole of sensitizing dye, 10 moles of secondary acidgenerator, 4.5 moles of superacid precursor and 0.5 mole of Ph-[DYE-H]⁺.After infra-red analysis to determine the amount of Ph-[DYE-H]⁺, theinfra-red irradiation may be adjusted so that, in exposed areas, only afurther 0.4 mole of superacid precursor is decomposed by the dye. Thus,after the infra-red irradiation, the medium will contain 0.9 mole of theprotonated product in exposed areas and 0.5 mole of the protonatedproduct in unexposed areas.

If no change were made in the ultra-violet irradiation step describedabove with reference to Table 1, the results would be disastrous, sincegeneration of a further 0.75 mole of acid in the unexposed areas wouldcause the acid concentration to exceed the threshold level, and thesecondary acid generator would decompose in both the exposed andunexposed areas. Accordingly, based upon the results of the infra-redanalysis, the ultra-violet irradiation is adjusted so that only (say)0.4 mole of acid are decomposed in the exposed and unexposed areas.Accordingly, after the ultra-violet irradiation, the exposed areascontain 1.3 moles of acid (0.3 mole above threshold level) in theexposed areas and 0.9 mole (still below threshold level) in theunexposed areas. The slight reduction in the amount of unbufferedsuperacid in the exposed areas (0.3 mole, versus 0.5 mole in Table 1)will not significantly affect the results of the heating step, and theoverall result of the imaging process will be unchanged.

For similar reasons, the present process is also relatively insensitiveto variations in infra-red radiation, such as those caused by variationsin laser output, variations between individual lasers in a laser diodearray used to form the imaging beam, timing errors in laser drivers,etc. For example, in the process shown in Table 1, the infra-redirradiation causes decomposition of 0.75 mole of superacid precursor. Ifthe infra-red radiation delivered to the imaging medium varies by ±20%,some exposed areas will experience decomposition of 0.6 mole ofsuperacid precursor, while others will experience decomposition of 0.9mole. After ultra-violet irradiation, the concentration of unbufferedsuperacid in the exposed areas will vary from 0.15 to 0.6 moles. Inpractice, with appropriate control of the heating step, this range ofvariation in unbuffered superacid concentration will have minimaleffects on the final image.

Any of the known superacid precursors, for example diazonium,phosphonium, sulfonium and iodonium compounds, may be used in thisinvention, but iodonium compounds are preferred. Especially preferredsuperacid precursors are diphenyliodonium salts, specifically(4-octyloxyphenyl)phenyliodonium hexafluorophosphate andhexafluoroantimonate and bis(n-dodecylphenyl)iodoniumhexafluoroantimonate.

Any infra-red dye capable of sensitizing decomposition of the superacidprecursor with the production of superacid may be used in the presentprocess. Preferably, the infra-red dye is a squarylium dye, sincesquarylium dyes tend to have high infra-red extinction coefficients,have long singlet excited state lifetimes (which assists the electrontransfer reactions upon which the present process depends), show littletendency to aggregate in polymeric films, and have low visibleabsorptions. Examples of infra-red dyes useful in the present processare:

a) dyes comprising an inner salt of a compound of the formula:

    Q.sup.1 =Z--Q.sup.2

wherein:

Q¹ is a 4-(benz[b]-4H-pyrylium)methylidene,4-(benz[b]-4H-thiopyrylium)methylidene or4-(benz[b]-4H-selenopyrylium)methylidene grouping;

Z is a 1,3-(2-hydroxy-4-oxo-2-cyclobutylidene) hydroxide or1,3-(2-hydroxy-4,5-dioxo-2-cyclopentylidene) hydroxide ring; and

Q² is a 4-(benz[b]-4H-pyran-4-ylidene)methyl,4-(benz[b]-4H-thiopyran-4-ylidene)methyl or4-(benz[b]-4H-selenopyran-4-ylidene)methyl grouping;

wherein at least one of the groupings Q¹ and Q² carries at its2-position a substituent in which a non-aromatic carbon atom is bondeddirectly to the benzpyrylium, benzthiopyrylium or benzselenopyryliumnucleus, subject to the proviso that if said 2-substituent contains anaromatic nucleus, this aromatic nucleus is not conjugated with thebenzpyrylium, benzthiopyrylium or benzselenopyrylium nucleus to which itis attached (see U.S. application Ser. No. 08/126,427, filed Sep. 24,1993 in the names of Stephen J. Telfer et al., and assigned to the sameassignee as the present application, and the corresponding InternationalApplication No. PCT/US91/08695, Publication No. WO 92/09661);

b) squarylium compounds of the formula: ##STR1## in which Q¹ and Q² areeach a chromophoric group having an unsaturated system conjugated withthe squarylium ring and such that in the compounds of formulae Q¹ CH₂ R¹and Q² CH₂ R² the methylene hydrogens are active hydrogens, R¹ and R²are each independently a hydrogen atom or an aliphatic or cycloaliphaticgroup, and R³ and R⁴ are each independently a hydrogen atom, or analiphatic, cycloaliphatic, aromatic or heterocyclic group, or one of R³and R⁴ is a hydrogen atom and the other is an organosulfonyl group, orR³ and R⁴ together with the intervening nitrogen atom form acycloaliphatic or aromatic ring (see U.S. Pat. No. 5,227,498 and thecorresponding International Application No. PCT/US92/09992, PublicationNo. WO 93/09956); and

c) squarylium compounds of the formula: ##STR2## in which: Q¹ and Q² areeach a chromophoric group having an unsaturated system conjugated withthe squarylium ring and such that in the compounds of formulae Q¹ CH₂ R¹and Q² CH₂ R² the methylene hydrogens are active hydrogens;

R¹ and R² are each independently a hydrogen atom or an aliphatic orcycloaliphatic group; and

R³, R⁴ and R⁵ are each independently a hydrogen atom, or an aliphatic,cycloaliphatic, aromatic or heterocyclic group, or anelectron-withdrawing group able to lower the electron density at thecarbon atom to which it is attached, subject to the provisoes that:

two of R³, R⁴ and R⁵ may form a divalent group of which a single atom isdouble bonded to the carbon atom to which the two groups are attached,or all three of R³, R⁴ and R⁵ may form a trivalent group of which asingle atom is triple bonded to the carbon atom to which the threegroups are attached, or

two of R³, R⁴ and R⁵ may, together with the carbon atom to which theyare attached, form a ring, or all three of R³, R⁴ and R⁵ may, togetherwith the carbon atom to which they are attached, form an unsaturatedring

(see U.S. Pat. No. 5,227,499 and the corresponding InternationalApplication No. PCT/US92/09992, Publication No. WO 93/09956).

Any secondary acid generator which is capable of superacid-catalyzedbreakdown to give a second acid may be used in the present process. Onepreferred group of secondary acid generators are3,4-disubstituted-cyclobut-3-ene-1,2 diones (hereinafter for conveniencereferred to as "squaric acid derivatives") capable of generating squaricacid or an acidic derivative thereof, since squaric acid and its acidicderivatives are strong acids well suited to effecting color changes orother effects (for example, polymerization or depolymerizationreactions) in acid-sensitive materials. Especially preferred squaricacid derivatives are those in which there is bonded to the squaric acidring, via an oxygen atom, an alkyl or alkylene group, a partiallyhydrogenated aryl or arylene group, or an aralkyl group. Theacid-catalyzed decomposition of these squaric acid derivatives causesreplacement of the original alkoxy, alkyleneoxy, aryloxy, aryleneoxy oraralkoxy group of the derivative with a hydroxyl group, therebyproducing squaric acid or an acidic squaric acid derivative having onehydroxyl group.

The exact mechanism by which squaric acid or an acidic derivativethereof is formed in the present process may vary depending upon thetype of squaric acid derivative employed. In some cases, for exampledi-t-butyl squarate, one or both groups attached via oxygen atoms to thesquaric acid ring may thermally decompose to yield an alkene or arene,thereby converting an alkoxy or aryloxy group to a hydroxyl group andforming the squaric acid or acidic derivative thereof. In other cases,for example 3-amino-4-(p-vinylbenzyloxy)cyclobut-3-ene-l,2-dione, thereis no obvious mechanism for formation of a corresponding alkene orarene, and it appears that the mechanism of acid formation is migrationof the vinylbenzyl carbocation or similar group to a different positionwithin the molecule (probably to the amino group), and protonation ofthe remaining oxygen atom to form a hydroxyl group at the position fromwhich the group migrates. In other cases, neither of these pathways ispossible. However, in all cases the net effect is the replacement of thealkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group present inthe original derivative with a hydroxyl group to form squaric acid or anacidic derivative thereof.

Those skilled in the art of organic chemistry will appreciate that thesusceptibility to thermal decomposition of the squaric acid derivativespreferred for use in the present process is related to the stability ofthe cation which is produced from the ester grouping during thedecomposition process. Although the stability of specific cations may beinfluenced by a variety of factors, including steric factors, which maybe peculiar to a particular ester, in general it may be stated that thesquaric acid esters preferred for use in the present process are:

(a) primary and secondary esters of squaric acid in which the α-carbonatom (i.e, the carbon atom bonded directly to the --O-- atom of thesquarate ring) bears a non-basic cation-stabilizing group. Thiscation-stabilizing group may be, for example, an sp² or sp hybridizedcarbon atom, or an oxygen atom;

(b) tertiary esters of squaric acid in which the α-carbon atom does nothave an sp² or sp hybridized carbon atom directly bonded thereto; and

(c) tertiary esters of squaric acid in which the α-carbon atom does havean sp² or sp hybridized carbon atom directly bonded thereto, providedthat this sp² or sp hybridized carbon atom (or at least one of these sp²or sp hybridized carbon atoms, if more than one such atom is bondeddirectly to the α-carbon atom) is conjugated with anelectron-withdrawing group. It will be apparent to skilled organicchemists that, provided one of the aforementioned types of estergroupings is present in the squaric acid derivative to produce onehydroxyl group after thermal decomposition, the group present in placeof the other hydroxyl group of squaric acid is of little consequence,provided that this other group does not interfere with the thermaldecomposition. Indeed, the wide variation possible in this other grouphas the advantage that this group can be varied to control otherproperties of the derivative, for example its compatibility with othercomponents of the imaging medium, or its solubility in solvents used toform coating solutions used in the preparation of the imaging medium.

Examples of squaric acid derivatives useful in the present processesinclude:

(a) those of the formula: ##STR3## in which R¹ is an alkyl group, apartially hydrogenated aromatic group, or an aralkyl group, and R² is ahydrogen atom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino,alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino,dialkylphosphoxy or trialkylsilyl group, subject to the proviso thateither or both of the groups R¹ and R² may be attached to a polymer.Among the derivatives of Formula I, especially preferred groups arethose in which (a) R¹ is an unsubstituted or phenyl substituted alkylgroup containing a total of not more than about 20 carbon atoms, and R²is an alkyl group containing not more than about 20 carbon atoms, or aphenyl group (which may be substituted or unsubstituted); and (b) R¹ isa benzyl group and R² is an amino group.

(b) those of the formula: ##STR4## in which R¹ and R³ independently areeach an alkyl group, a partially hydrogenated aryl group or an aralkylgroup, subject to the proviso that either or both of the groups R¹ andR³ may be attached to a polymer. Among the derivatives of Formula II, anespecially preferred group are those in which R¹ and R³ are eachindependently an unsubstituted or phenyl substituted alkyl groupcontaining a total of not more than about 20 carbon atoms. Specificpreferred compounds of Formula II are those in which R¹ and R³ are eacha tertiary butyl group, a benzyl group, an α-methylbenzyl group or acyclohexyl group, namely di-tertiary butyl squarate, dibenzyl squarate,bis(α-methylbenzyl) squarate and dicyclohexyl squarate.

(c) those of the formula: ##STR5## in which n is 0 or 1, and R⁴ is analkylene group or a partially hydrogenated arylene group. Among thederivatives of Formula III, an especially preferred group are those inwhich n is 1 and R⁴ is an alkylene group containing not more than about12 carbon atoms.

(d) those having at least one unit of the formula: ##STR6## in which nis 0 or 1, R⁵ is an alkylene or partially hydrogenated arylene group. Inaddition to the fragmentable groups R⁵, the compounds may also containone or more units in which a non-fragmentable group is attached to asquarate ring, directly or via an oxygen atom.

The squaric acid derivatives of Formula IV include not only highpolymers, but also dimers, trimers, tetramers, etc. including at leastone of the specified units. The terminating groups on the derivatives ofFormula IV may be any of groups OR¹ or R² discussed above with referenceto Formula I. Thus, for example, Formula IV includes the squaric aciddimer derivative of the formula: ##STR7##

The squaric acid derivatives of Formulae I and II are usually monomeric.However, these derivatives of Formulae I and II can be incorporated intopolymers by having at least one of the groups R¹, R² and R³ attached toa polymer. Attachment of the squaric acid derivatives to a polymer inthis manner may be advantageous in that it may avoid incompatibilityand/or phase separation which might occur between a monomeric squaricacid derivative of Formula I or II and a polymeric binder needed in animaging medium.

The attachment of the groups R¹, R² and R³ to a polymer may be effectedin various ways, which will be familiar to those skilled in the art ofpolymer synthesis. The squaric acid derivatives may be incorporated intothe backbone of a polymer, for example in a polymer similar to the dimerof the formula given above. Alternatively, the squaric acid derivativesmay be present as sidechains on a polymer; for example, one of thegroups R¹, R² and R³ could contain an amino group able to react with apolymer containing a carboxyl groups or derivatives thereof to form anamide linkage which would link the squaric acid derivative as asidechain on to the polymer, or these groups may contain unsaturatedlinkages which enable the squaric acid derivatives to be polymerized,either alone or in admixture with other unsaturated monomers.

In the present process, it is generally undesirable to form substantialquantities of gas during the superacid-catalyzed decomposition of thesquaric acid derivative (or other secondary acid generator) since suchgas may distort the medium containing the squaric acid derivative orform vesicles therein, and such distortion or vesicle formation mayinterfere with proper image formation. Accordingly, if the decompositionof the squaric acid derivative yields an alkene, it is desirable thatthe groups R¹, R³, R⁴ and R⁵ be chosen so that this alkene is a liquidat 20° C., and preferably higher, since some heating of the alkene willinevitably occur during the superacid-catalyzed decomposition. In somecases, however, the alkene liberated may be sufficiently soluble in themedium containing the squaric acid derivative that liberation of ahighly volatile alkene will not result in distortion of, or vesicleformation in, the medium.

Another preferred group of secondary acid generators for use in thepresent process are oxalic acid derivatives which undergosuperacid-catalyzed breakdown to give oxalic acid or an acidicderivative thereof, for example an oxalic acid hemiester. Althoughoxalic acid and its acidic derivatives are not quite such strong acidsas squaric acid and its acidic derivatives, oxalic acid and itsderivatives are sufficiently strong acids for most purposes for whichsecondary acids are required in the present process. Also, oxalic acidderivatives are, in general, less costly than squaric acid derivatives.

The types of oxalic acid derivatives preferred for use in the presentprocess are rather more diverse in structure than the squaric acidderivatives, and the choice of oxalic acid derivative for any specificprocess may be governed more by the thermal breakdown properties of thederivative than its exact chemical structure; in general, for practicalreasons such as the limited temperature range to which other componentsof the imaging medium may safely be exposed, it is preferred that theoxalic acid derivative be one which begins to decompose thermally at atemperature in the range of about 140° to about 180° C., as measured bydifferential scanning calorimetry in a nitrogen atmosphere at a 10°C./minute temperature ramp, in the absence of any catalyst. Since thepresence of a superacid catalyst lowers the thermal decompositiontemperature of oxalic acid derivatives by at least about 20° C. andpotentially significantly more, derivatives which decompose uncatalyzedat about 140° to about 180° C., will, in the presence of superacid,decompose at temperatures as low as about 65° C., temperatures to whichother components of the imaging medium can in general be exposed.

The factors affecting the ability of the oxalic acid derivatives toundergo superacid-catalyzed thermal decomposition are similar to thoseaffecting the ability of the aforementioned squaric acid derivatives toundergo the same reaction, and thus the preferred ester groups are ofthe same types. Accordingly, preferred oxalic acid derivatives for usein the present process include:

(a) primary and secondary esters of oxalic acid in which the α-carbonatom (i.e, the carbon atom bonded directly to the --O-- atom of theoxalate grouping) bears a non-basic cation-stabilizing group. Thiscation-stabilizing group may be, for example, an sp² or sp hybridizedcarbon atom, or an oxygen atom;

(b) tertiary esters of oxalic acid in which the α-carbon atom does nothave an sp² or sp hybridized carbon atom directly bonded thereto; and

(c) tertiary esters of oxalic acid in which the α-carbon atom does havean sp² or sp hybridized carbon atom directly bonded thereto, providedthat this sp² or sp hybridized carbon atom (or at least one of these sp²or sp hybridized carbon atoms, if more than one such atom is bondeddirectly to the α-carbon atom) is conjugated with anelectron-withdrawing group.

(d) an ester formed by condensation of two moles of an alcohol with thebis(hemioxalate) of a diol, provided that the ester contains at leastone ester grouping of types (a), (b) or (c) above. One example of anester of this type is that of the structure: ##STR8## which can beregarded as formed from two moles of menthol(2-methylethyl-4-methylcyclohexanol) and one mole of thebis(hemioxalate) of 1,6-bis-(4-hydroxymethylphenoxy)hexane. Since thestructure of the central residue of the diol in such esters can varywidely, the solubility and other properties of the esters can be "tuned"as required for compatibility with other components of the imagingmedium, while the nature of the end groups, which undergo theacid-forming thermal decomposition, can be varied independently of thenature of the central residue.

(e) polymeric oxalates derived from polymerization of oxalate estershaving an ethylenically unsaturated group, provided that the estercontains at least one ester grouping of types (a), (b) or (c) above. Aswith the squaric acid derivatives discussed above, use of a polymericoxalate rather than a monomeric one may be advantageous in that it mayavoid incompatibility and/or phase separation which might occur betweena monomeric derivative and a polymeric binder needed in an imagingmedium. Use of a polymeric derivative also tends to inhibit diffusion ofthe oxalate through the imaging medium during storage prior to imaging.Although polymeric oxalates can be formed in other ways, at present weprefer to form such oxalates by first forming an oxalate ester in whichone of the ester groupings comprises an ethylenically unsaturated group,and then polymerizing this ester using a conventional free radicalpolymerization initiator, for example azobis(isobutyronitrile) (AIBN).The ethylenically unsaturated group is conveniently an acrylate ormethacrylate group, while the other ester grouping in the monomericoxalate can be any of the types discussed above.

(f) Condensation polymers of oxalates, provided that the ester containsat least one ester grouping of types (a), (b) or (c) above. This type ofpolymer also possesses the advantages discussed under (e) above.

As already mentioned, the present process may be used for variouspurposes, such as triggering of an acid-catalyzed chemical reaction (forexample, polymerization or depolymerization reactions). When the presentprocess is used for image formation, simultaneously with or subsequentto the heating step, the second acid is contacted with an acid-sensitivematerial which changes color in the presence of the second acid. (Itwill be appreciated that the "color change" involved in such an imagingprocess need not be a visible color change. If, for example, the presentprocess is used to provide security markings intended to bemachine-readable, the "color change" could be a change in absorptionfrom one non-visible wavelength to another, such that it can be detectedby the appropriate machine-reading device.)

The acid-sensitive material used in the process of the present inventionmay be any material which undergoes a color change in the presence ofthe second acid. Thus any conventional indicator dye may be used as theacid-sensitive material, as may the leuco dyes disclosed in theaforementioned U.S. Pat. Nos. 4,602,263; 4,720,449 and 4,826,976, whichare also sensitive to acid.

The exposure of the medium to the actinic (typically infra-red)radiation of the first wavelength can be effected in any of the waysconventionally used for exposing media to the same type of radiation. Insome cases, it may be convenient to employ a laser of the appropriatewavelength, since the use of a laser is a convenient way to record dataas an image pattern in response to transmitted signals, such asdigitized information.

Some imaging media of the present invention (for example those intendedfor use as photoresists and containing polymerizable monomers oroligomers or depolymerizable polymers) may comprise only a single layercontaining all the components of the imaging medium. However, mediacontaining a secondary acid generator and an acid-sensitive materialdesirably comprise two separate layers or phases, so that, prior to theheating, the acid-sensitive material is present in a layer or phaseseparate from the layer or phase containing the superacid precursor andthe secondary acid generator, and following the generation of the secondacid from the secondary acid generator, the two layers or phases aremixed, thereby effecting the color or other change in the acid-sensitivematerial.

In principle, the mixing of the acid-sensitive material with thesuperacid precursor, sensitizing dye and secondary acid generator shouldbe effected after the generation of the second acid from the secondaryacid generator. However, in practice if the superacid precursor,sensitizing dye and secondary acid generator are present in one layer ofa two-layer imaging medium, and the acid-sensitive material in the otherlayer of the medium, these two layers being such that their diffusiblecomponents mix on heating, both the generation of the second acid andthe mixing of the two layers may be effected in a single heating step,since the superacid-catalyzed decomposition of the secondary acidgenerator will typically be essentially complete before mixing of thetwo layers becomes significant.

When a two-layer structure is used, it is not necessary that the twolayers be affixed to one another before imaging. The production ofunbuffered superacid and second acid in exposed regions effected by thepresent processes are "permanent" chemical changes, and hence it ispossible to delay contacting the exposed medium with an acid-sensitivematerial for a substantial time. (Obviously, excessive delay may reducethe quality of an image produced by allowing superacid or second acid todiffuse from exposed into unexposed areas of the medium.) Accordingly,the two layers of the imaging medium may be laminated together after thesecond irradiation. However, in general it is most convenient to formthe two layers by coating one on the other, or laminating the two layerstogether before imaging, since in this way only a single sheet ofmaterial has to handled during the imaging process. Since it isimportant that the two layers not mix prematurely, if the two layers areto be coated successively on to a support, it is usually desirable tocoat one layer from an aqueous medium and the other from a non-aqueousmedium. Typically, the layer containing the superacid precursor iscoated from an organic solution and the layer containing anacid-sensitive leuco dye or other material is coated from an aqueousdispersion.

As already mentioned above with reference to Table 1 and FIG. 1, priorto the heating step, the acid-sensitive material may be in admixturewith an amount of a basic material insufficient to neutralize all thesecond acid liberated by the secondary acid generator during theheating, so that the second acid liberated by the secondary acidgenerator during the heating neutralizes all of the basic material andleaves excess second acid sufficient to effect the change in theacid-sensitive material. The provision of this basic material serves to"soak up" minor amounts of acid which may be generated in unexposedareas after exposure due, for example, to slow decomposition of thesuperacid precursor/sensitizing dye mixture during protracted storage.Since obviously the basic material cannot be allowed to contact thesuperacid present after the second irradiation but prior to the heatingstep, desirably the acid-sensitive material is present in a layer orphase separate from the layer or phase containing the superacidprecursor and the secondary acid generator and, following the generationof the second acid, the two layers or phases are mixed, therebyeffecting the change in the acid-sensitive material.

In addition to the two aforementioned layers or phases containing thesuperacid precursor, sensitizing dye, secondary acid generator andacid-sensitive material, the imaging media of the present invention maycomprise a support and additional layers, for example, a subbing layerto improve adhesion to the support, acid-impermeable interlayers forseparating multiple imaging layers from one another, an anti-abrasivetopcoat layer, and other auxiliary layers.

The support employed may be transparent or opaque and may be anymaterial that retains its dimensional stability at the temperature usedfor image formation. Suitable supports include paper, paper coated witha resin or pigment, such as, calcium carbonate or calcined clay,synthetic papers or plastic films, such as polyethylene, polypropylene,polycarbonate, cellulose acetate and polystyrene. The preferred materialfor the support is a polyester, desirably poly(ethylene terephthalate).

Usually the layer containing the superacid precursor, sensitizing dye,and secondary acid generator, and the layer containing theacid-sensitive material, will each also contain a binder; typicallythese layers are formed by combining the active materials and the binderin a common solvent, applying a layer of the coating composition to thesupport and then drying. Rather than a solution coating, the layer maybe applied as a dispersion or an emulsion. The coating composition alsomay contain dispersing agents, plasticizers, defoaming agents, coatingaids and materials such as waxes to prevent sticking.

The binder used for the layer(s) in which superacid is to be generatedmust of course be non-basic, such that the superacid is not buffered bythe binder. Examples of binders that may be used includestyrene-acrylonitrile copolymers, polystyrene, poly(α-methylstyrene),copolymers of styrene and butadiene, poly(methyl methacrylate),copolymers of methyl and ethyl acrylate, poly(vinyl butyral),polycarbonate, poly(vinylidene chloride) and poly(vinyl chloride). Itwill be appreciated that the binder selected should not have any adverseeffect on the superacid precursor, sensitizing dye, secondary acidgenerator or the acid-sensitive material incorporated therein. Also, thebinder should be heat-stable at the temperatures encountered during theheating step and should be transparent so that it does not interferewith viewing of the image. The binder must of course transmit theactinic radiation used in the exposure steps.

The squaric acid derivatives preferably used as acid generators in theprocess of the present invention can be prepared by known methods, suchas those described in U.S. Pat. No. 4,092,146 and Tetrahedron Letters(1977), 4437-38, and 23,361-4, and Chem. Ber. 121,569-71 (1988) and 113,1-8 (1980). In general, the diesters of Formula II can be prepared byreacting disilver squarate with the appropriate alkyl halide(s),preferably the alkyl bromides. The ester groupings may be varied byroutine transesterification reactions, or by reacting the diacidchloride of squaric acid with an appropriate alcohol or alkoxide.

The derivatives of Formula I in which R² is an alkyl, cycloalkyl,aralkyl or aryl group can be prepared from derivatives of Formula II bythe synthesis shown in FIG. 2. The diester of Formula II is firstcondensed with a compound containing a negatively charged species R² ;this compound is normally an organometallic compound, and preferably anorganolithium compound. The reaction adds the -R² group to one of theoxo groups of the diester to produce the squaric acid derivative ofFormula VI; to avoid disubstitution into both oxo groups, not more thanthe stoichiometric amount of the organometallic reagent should be used.

After being separated from unreacted starting material and otherby-products, the squaric acid derivative VI is treated with an acid, forexample hydrochloric acid, to convert it to the desired squaric acidderivative I. Although it is possible to simply add acid to the reactionmixture resulting from the treatment of the diester with theorganometallic reagent, this course is not recommended, since thesquaric acid derivative I produced may be contaminated with unreacteddiester, and the diester and squaric acid derivative I are so similarthat it is extremely difficult to separate them, even by chromatography.

It will be appreciated that the synthesis shown in FIG. 2 may bemodified in various ways. If, for example, the nature of the group R¹desired in the final compound of Formula I is such that it would reactwith the organometallic reagent, the reactions shown in FIG. 2 may becarried out with a diester in which the ester groupings do not containthe group R¹, and the final product of Formula I may be subjected totransesterification or other reactions to introduce the group R¹.

The derivatives of Formula I in which R² is an amino, alkylamino ordialkylamino group can be prepared by similar methods from squaric aciddiesters. For example, as illustrated in the Examples below, reaction ofbis(4-vinylbenzyl) squarate with methylamine gives3-amino-4-(p-vinylbenzyloxy)cyclobut-3-ene-1,2-dione. Analogous methodsfor the synthesis of the other compounds of Formula I will readily beapparent to those skilled in the art of organic synthesis.

The forms of the squaric acid derivatives of Formulae I and II in whichat least one of R¹, R² and R³ is attached to a polymer may be preparedby reactions analogous to those used to prepare the monomericderivatives of Formulae I and II, for example by treating a polymercontaining appropriate alkoxide groups with the diacid chloride or amonoester monoacid chloride of squaric acid. Alternatively, thesepolymer-attached derivatives may be prepared by transesterification, forexample by treating a polymer containing esterified hydroxyl groups witha monomeric squaric acid derivative of Formula I or II. Other methodsfor attachment of these derivatives to polymers, or inclusion of thesederivatives into polymer backbones, have already been discussed above.

The derivatives of Formula III may be prepared by transesterificationfrom derivatives of Formula II, or another squaric acid diester, and theappropriate diol.

The monomeric oxalic acid derivatives useful in the present process canbe prepared by routine esterification techniques which will be familiarto those skilled in organic synthesis, and several Examples of suchtechniques are exemplified in detail below. The preparation of polymericoxalic acid derivatives has already been discussed.

A preferred embodiment of the invention will now be described, though byway of illustration only, with reference to FIG. 3 of the accompanyingdrawings, which shows a schematic cross-section through an imagingmedium (generally designated 10) of the invention as the image thereinis being fixed by being passed between a pair of hot rollers 12.

The imaging medium 10 comprises a support 14 formed from a plastic film.Typically the support 14 will comprise a polyethylene terephthalate film3 to 10 mils (76 to 254 mμ) in thickness, and its upper surface (in FIG.3) may be treated with a sub-coat, such as are well-known to thoseskilled in the preparation of imaging media, to improve adhesion of theother layers to the support.

On the support 14 is disposed an acid-generating layer 16 comprising asuperacid precursor, an infra-red sensitizing dye and a secondary acidgenerator, which undergoes a superacid-catalyzed thermal decompositionto form a second acid. On the opposed side of the acid-generating layer16 from the support 14 is disposed a color-forming layer 18 comprisingan acid-sensitive material, which changes color in the presence of anacid, and a small amount of a base. The acid-generating layer 16 and theimaging layer 18 both contain a binder having a glass transitiontemperature substantially above room temperature.

Finally, the imaging medium comprises an abrasion-resistant topcoat 20.

The imaging medium 10 may be formed by coating the layers 16, 18 and 20on to the support 14. Alternatively, for example, the layers 16 and 18may be coated on to the support 14, and the topcoat 20 laminated on tothe resultant structure.

The imaging medium 10 is exposed by writing on selected areas of themedium with an infra-red laser; this exposure may be effected throughthe support 14, as indicated by the arrow 22 in the drawing(alternatively, exposure could be effected through the topcoat 20).Within the exposed regions of the acid-generating layer 16, the exposureto infra-red radiation causes breakdown of the superacid precursor withthe formation of the corresponding superacid buffered by the sensitizingdye, as described above. After this infra-red exposure, the imagingmedium 10 is passed beneath a mercury lamp and given a blanketultraviolet exposure to produce unbuffered superacid in the infra-redexposed areas, and then passed between the heated rollers 12. The heatapplied by the rollers 12 causes the superacid present in the exposedregions of the acid-generating layer 16 to cause catalytic breakdown ofthe secondary acid generator therein, thereby causing formation of aquantity of second acid substantially larger than the quantity ofsuperacid originally generated by the ultra-violet radiation. The heatand pressure applied by the rollers 12 also raise the color-forminglayer 18 and the acid-generating layer 16 above their glass transitiontemperatures, thereby causing the components dispersed in these twolayers to become intermixed so that, in exposed regions, the second acidproduced in the acid-generating layer 16 effects the color change of theacid-sensitive material, thereby forming an image.

The imaging medium 10 shown in FIG. 3 will produce monochrome images. Aswill readily be apparent to those skilled in the imaging art, thisimaging medium 10 may readily be modified to produce full color imagesby including two or more additional pairs of color-forming layers 18 andacid-generating layers 16, with acid-impermeable interlayers providedbetween each adjacent pair of layers, the interlayers having a glasstransition temperature sufficiently high that it is not exceeded duringpassage of the medium between the rollers 12, so that the interlayersprevent mixing of adjacent pairs of layers 16 and 18. Typically, amulticolor medium will comprise three pairs of color-forming layers 18and acid-generating layers 16 arranged to produce yellow, cyan andmagenta images, as in conventional multicolor imaging media. Theacid-generating layers 16 in such a medium will contain infra-redsensitizing dyes absorbing at differing wavelengths so that the threecolor-forming layers can be imaged independently of one another usingthree infra-red lasers of differing wavelengths. It should be noted thatonly the infra-red sensitizing dyes need differ among the plurality ofacid-generating layers; conveniently, all the acid-generating layers canmake use of the same superacid precursor and secondary acid generator.

The following Examples are now given, though by way of illustrationonly, to show details of preferred reagents, conditions and techniquesused in the process and imaging medium of the present invention.

EXAMPLES 1-11

Preparation of squaric acid derivative secondary acid generators

3,4-Bis(t-butoxy)cyclobut-3-ene-1,2-dione ("bis t-butyl squarate";hereinafter referred to as "Compound A") used in certain Examples belowwas prepared as described in E. V. Dehmlow et al., Chem. Bet. 113, 1-8(1980). 3,4-Bis(benzyloxy)cyclobut-3-ene-1,2-dione ("dibenzyl squarate";hereinafter referred to as "Compound B") used in certain Examples belowwas prepared as described in N. Islam et al, Tetrahedron 43, 959-970(1987). Silver squarate was prepared as described in S. Cohen et al., J.Am. Chem. Soc., 88, 5433 (1966).

Example 1

Preparation of bis(3-bromo-2,3-dimethylbut-2-yl)squarate

This Example illustrates the preparation of3,4-bis(3-bromo-2,3-dimethylbut-2-oxy )-cyclobut-3-ene-1,2-dione("bis(3-bromo-2,3-dimethylbut-2-yl) squarate"), the compound of FormulaII in which R¹ and R³ are each a 3-bromo-2,3-dimethylbut-2-yl group.

Silver squarate (1.0 g, 3.0 mmole) was added to a solution of2,3-dibromo-2,3-dimethylbutane (1.0 g, 4.0 mmole) in dry ether (3 mL) atroom temperature. The suspension became warm, and was cooled by a waterbath at room temperature. After six hours' stirring, the precipitateremaining was removed by filtration, and washed with ether. The combinedether extracts were concentrated, and the crude product obtainedtherefrom was purified by flash chromatography on silica gel with 1:3ether/hexanes as eluent to give the diester (140 mg, 11% yield) as awhite powder which decomposed at 131°-132° C. The structure of thecompound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 2

Preparation of 3-t-butoxy-4-phenylcyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3-t-butoxy-4-phenyl-cyclobut-3-ene-1,2-dione, the compound of Formula Iin which R¹ is a tertiary butyl group and R² is a phenyl group.

Phenyl magnesium bromide (4.6 mL of a 1.0M solution in THF, 4.6 mmole)was added dropwise over a period of 5 minutes to a solution ofdi-t-butyl squarate (1.0 g, 4.42 mmole) in dry ether (10 mL) at -78° C.under nitrogen. After 30 minutes, the reaction mixture was warmed to 0°C., and stirred at this temperature for an additional one hour. Water(10 mL) and ether (10 mL) were then added to the reaction mixture andthe layers were separated. The aqueous layer was extracted twice withdichloromethane. The combined organic layers were dried over magnesiumsulfate and concentrated, to give a yellow oil (1.43 g), whichcrystallized. The resultant material was dissolved in dichloromethane(25 mL) and concentrated hydrochloric acid (4 drops) was added, withstirring, to this solution at room temperature. After 30 minutes, afurther four drops of concentrated hydrochloric acid were added.Dichloromethane (25 mL) was added, and the resultant solution was washedwith a saturated solution of sodium bicarbonate and then with brine,dried over magnesium sulfate, and concentrated. The crude product thusobtained was purified by flash chromatography on silica gel with tolueneas eluent. The chromatographed material was further purified byrecrystallization from toluene/hexanes to give the desired monoester asyellow crystals (142 mg, 14% yield) which decomposed at 105°-110° C. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 3

Preparation of 3,4-bis(α-methylbenzyloxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3,4-bis(α-methyl-benzyloxy)-cyclobut-3-ene-1,2-dione("bis(α-methylbenzyl) squarate"), the compound of Formula II in which R¹and R³ are each an α-methylbenzyl group.

1-Bromo-1-phenylethane (3.1 g, 16.8 mmole) was added dropwise to asuspension of silver squarate (2.5 g, 7.62 mmole) in dry ether (40 mL)at 0° C. After the addition was complete, the reaction mixture wasallowed to warm to room temperature and was stirred for four hours inthe dark. The solid remaining after this time (silver bromide) wasremoved by filtration and washed with more ether. The combined ethersolutions were washed with a saturated solution of sodium bicarbonateand dried over sodium sulfate. Evaporation of the solvent was followedby purification by flash chromatography on silica gel with 0-60%ether/hexanes as eluant to give the desired diester (394 mg, 16% yield)as a colorless oil. The diester was obtained as a mixture ofdiastereoisomers which were not separable by this type ofchromatography. The structure of the diester was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 4

Preparation of 3,4-bis(p-methylbenzyloxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3,4-bis(p-methyl-benzyloxy)-cyclobut-3-ene-1,2-dione("bis(p-methylbenzyl)squarate"), the compound of Formula II in which R¹and R³ are each a p-methylbenzyl group.

Triethylamine (0.93 g, 9.2 mmole) was added to a stirred suspension ofsquaric acid (0.5 g, 4.38 mmole) in chloroform (10 mL) and the resultantsolution was cooled with an ice/water bath. A solution ofα-bromo-p-xylene (2.03 g, 11.0 mmole) in chloroform (10 mL) was thenadded dropwise over a period of 30 minutes. After this time, the coolingbath was removed and the solution was held at room temperature for 4.5hours. The reaction mixture was then diluted with chloroform (20 mL),washed successively with a saturated aqueous solution of sodiumbicarbonate (2×20 mL) and saturated brine (20 mL), dried over magnesiumsulfate and concentrated under reduced pressure. The resultant oil wasfurther purified by partition between ether (50 mL) and saturatedaqueous sodium bicarbonate (20 mL) and separation of the organic layer.The organic layer was washed successively with a saturated aqueoussolution of sodium bicarbonate (20 mL) and saturated brine (20 mL),dried over magnesium sulfate and concentrated under reduced pressure.The oil which resulted was crystallized from hot hexanes (20 mL) to givethe desired compound (300 mg, 21.3% yield) as off-white crystals. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 5

Preparation of 3,4-bis(cyclohexyloxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3,4-bis(cyclohexyloxy)-cyclobut-3-ene-1,2-dione ("dicyclohexylsquarate"), the compound of Formula II in which R¹ and R³ are each acyclohexyl group.

Cyclohexyl bromide (9.95 g, 61 mmole) was added dropwise over a periodof 20 minutes to a stirred suspension of silver squarate (4.0 g, 12.2mmole) in ether (80 mL) in the dark with ice/water cooling. The ice bathwas then removed and the reaction mixture was stirred overnight at roomtemperature, then filtered to remove silver bromide, and the residue waswashed with ether (2×20 mL). The ether solutions were combined andwashed successively with a saturated aqueous solution of sodiumbicarbonate (50 mL) and saturated brine (50 mL), dried over magnesiumsulfate and concentrated under reduced pressure to give the desiredcompound as a viscous oil which solidified upon storage in arefrigerator to give an off-white solid (0.55 g, 16% yield). Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 6

Preparation of 3-amino-4-(t-butoxy)-cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3-amino-4-(t-butoxy)-cyclobut-3-ene-1,2-dione, the compound of Formula Iin which R¹ is a tertiary butyl group and R² is an amino group.

A stream of ammonia gas was passed into a stirred solution of Compound A(0.7 g, 3.07 mmole) in methanol (40 mL) for 2 minutes. The solution wasthen allowed to stand at room temperature for 1 hour, during which timea small amount of insoluble material was precipitated. The sediment wasremoved by filtration, and the solvent was removed under reducedpressure to yield a yellow solid, which was washed with ether (2×50 mL)to remove starting material and butanol (0.16 g of impurities werecollected, after solvent evaporation). The solid which remained wasdissolved in dichloromethane (150 mL) and the solution was filtered.Removal of the solvent under reduced pressure yielded the desiredcompound as white crystals (0.25 g, 48% yield) which melted at 220°-225°C. The structure of this compound was confirmed by ¹ H NMR spectroscopy.

Example 7

Preparation of 4-hexyl-3-(p-vinyl-benzyloxy)cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of4-hexyl-3-(p-vinyl-benzyloxy)-cyclobut-3-ene-1,2-dione, the compound ofFormula I in which R² is a hexyl group and R¹ is an p-vinylbenzyl group.

Part A: Preparation of 2,3-dibutoxy-4-hexyl-4-hydroxycyclobut-2-en-1-one

Hexyl magnesium bromide (40 mL of a 2M solution in ether, 80.0 mmole)was added dropwise over a period of 45 minutes to a solution ofdi-n-butyl squarate in dry THF (150 mL) at -78° C. under nitrogen, andthe reaction mixture was held at that temperature for 1 hour. Thereaction mixture was then allowed to warm to room temperature arestirred for an additional 3 hours, after which time it was cooled usingan ice/water bath, and quenched by the addition of water (25 mL) addeddropwise over a period of 5 minutes. Saturated brine (300 mL) and ether(300 mL) were then added, the layers were separated, and the aqueouslayer was extracted with additional ether (300 mL). The ether extractswere combined and dried over magnesium sulfate, and the solvents wereremoved to give a golden oil (15.64 g) containing the desired product;this oil was used without further purification in Part B below.

Part B: Preparation of 3-hexyl-4-hydroxy-cyclobut-3-ene-1,2-dione

6N Hydrochloric acid (150 mL) was added in one portion to a stirredsolution of crude 2,3-dibutoxy-4-hexyl-4-hydroxycyclobut-2-en-1-one(15.1 g, prepared in Part A above) in THF (150 mL), and the resultantsolution was stirred at room temperature for 3 hours. The reactionmixture was then concentrated under reduced pressure to give a yellowsolid. To this solid was added water (100 mL), which was then removedunder reduced pressure. Toluene (100 mL) was similarly added and removedunder reduced pressure, and then dichloromethane (200 mL) was added tothe residue and the resultant solution was filtered and concentrated toproduce a yellow oil. Hexanes (200 mL) were added and the resultantsolution was cooled to induce crystallization. After recrystallizationfrom hexanes, the desired compound was isolated as tan crystals (4.28 g,33% yield over Parts A and B). The structure of this compound wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Part C: Preparation of4-hexyl-3-(p-vinylbenzyloxy)-cyclobut-3-ene-1,2-dione

Triethylamine (1.75 g, 17.3 mmole),2,6-di-t-butyl-4-methylphenol (aradical inhibitor, 0.7 mg, 3.4 μmol) and 4-vinylbenzyl chloride (5.04 g,33 mmole) were added, in that order, to a solution of3-hexyl-4-hydroxy-cyclobut-3-en-1,2-one (3.0 g, 16.5 mmole, prepared inPart B above) in chloroform (90 mL), and the resultant solution washeated at reflux for 7 hours. The solution was then cooled and allowedto stand overnight at room temperature, after which it was heated atreflux for a further 7 hours, then cooled and allowed to stand overnighta second time. The reaction mixture was then concentrated under reducedpressure, the residue dissolved in dichloromethane (150 mL), and theresultant solution washed with water (2×75 mL), dried over magnesiumsulfate and concentrated under reduced pressure to yield a yellow oil,which was purified by short-path distillation (to remove excess4-vinylbenzyl chloride) at 72°-74° C. and 1.7 mm Hg pressure. Theresidue from the distillation was purified by flash chromatography onsilica gel with dichloromethane as eluant to give the desired compound(1.23 g, 25% yield) as a golden oil. The structure of this compound wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 8

Preparation of3-methylamino-4-(p-vinyl-benzyloxy)cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of3-methylamino-4-(p-vinyl-benzyloxy)-cyclobut-3-ene-1,2-dione, thecompound of Formula I in which R² is an amino group and R¹ is ap-vinylbenzyl group.

Part A: Preparation of bis(4-vinylbenzyl) squarate

4-Vinylbenzyl chloride (13 g, 85 mmole) was added to a suspension ofsilver squarate (5.5 g, 48 mmole) in dry ether (100 mL), and theresultant mixture was stirred in the dark for 3 days. The reactionmixture was then filtered and the solvent removed under reducedpressure. The residue was taken up in dichloromethane and filteredthrough a short column of silica gel, then concentrated under reducedpressure, to yield the desired compound in a crude form, which was usedin Part B below without further purification.

Part B: Preparation of3-methylamino-4-(p-vinylbenzyloxy)-cyclobut-3-ene-1,2-dione

The crude product from Part A above was dissolved in ether (300 mL) andgaseous methylamine was bubbled through this ether solution for 1minute. The resultant mixture was allowed to stand for 5 minutes, thenthe precipitate which had formed was removed by filtration, redissolvedin chloroform and filtered through Celite (manufactured byJohns-Manville Corporation, Denver, Colo. 80217). The solvent wasremoved under reduced pressure to give the desired product (hereinaftercalled "Compound H") as a white solid, melting point 152° C. (3.5 g, 30%yield over Parts A and B). The structure of this compound was confirmedby ¹ H NMR spectroscopy.

Example 9

Preparation of copolymer of Compound H with lauryl methacrylate

This Example illustrates the preparation of a 1:1 w/w copolymer ofCompound H prepared in Example 8 above with lauryl methacrylate.

Compound H (1 g) and lauryl methacrylate (1 g) were dissolved in amixture of 2-propanol (30 mL) and ethanol (20 mL), and the resultantsolution was purged with nitrogen. Azoisobutyronitrile (0.01 g) was thenadded, and the solution was held at 65° C. overnight, during which timea precipitate (250 mg) formed. This precipitate was collected and shownby infra-red spectroscopy to contain squarate esters.

Example 10

Preparation of4-[5-[1,2-dioxo-3-hydroxycyclobut-3-en-4-yl]pent-1-yl]-3-hydroxycyclobut-3-ene-1,2-dione

Pentamethylene bis(magnesium bromide) (25 mL of a 0.5M solution in THF,12.5 mmole) was added dropwise over a period of 15 minutes to a solutionof dibutyl squarate (5.66 g, 25 mmole) in dry THF (50 mL) at -78° C.under a stream of nitrogen. The resulting suspension was stirred at -78°C. for 1 hour, then allowed to warm to room temperature and stirred fora further 2 hours. The homogeneous yellow solution which resulted wascooled to 0° C., and water (10 mL) was added dropwise over a period of 2minutes. After standing for 5 minutes, the solution was diluted with THF(50 mL) and washed with saturated sodium chloride solution (150 mL). Anemulsion was formed, which was separated by evaporative removal of THFand addition of dichloromethane (200 mL). The organic layer wasseparated and the aqueous layer was extracted with more dichloromethane(100 mL). The combined dichloromethane layers were dried over magnesiumsulfate and concentrated under reduced pressure to yield a golden oilwhich was shown by thin layer chromatography, on silica gel with 1:1ether/hexanes as eluent, to consist of five components.

This mixture was separated by flash chromatography on silica gel with1:1 ether/hexanes, followed by pure ether, as eluents. Each of the fivecomponents was examined by ¹ H NMR spectroscopy. The third and fourthcomponents (in order of elution from the column) were tentativelyassigned as4-[5-[1,2-dioxo-3-butoxy-cyclobut-3-en-4-yl]pent-1-yl]-3-butoxycyclobut-3-ene-1,2-dione(0.69 g) and2,3-dibutoxy-[5-[1,2-dioxo-3-butoxycyclobut-3-en-4-yl]pent-1-yl]-4-hydroxycyclobut-2-en-1-one(2.14 g).

A portion of the isolated fourth component (2.01 g) was dissolved in THF(20 mL), and the resultant solution was treated with 6M hydrochloricacid (20 mL). The two-phase mixture became warm, and after 15 minutesstirring was observed to have become homogeneous. After a further twohours stirring, the solution was concentrated to dryness under reducedpressure. Water (20 mL) was added, and removed by evaporation, in orderto drive off excess hydrogen chloride. The remaining water was removedby azeotropic distillation under reduced pressure withdichloromethane/acetone, to yield an off-white solid. This material waspurified by recrystallization from THF/ether to yield the desiredcompound as a tan powder (542 mg, 18% yield over two steps). Thestructure of this compound was confirmed by ¹ H and ¹³ C NMRspectroscopy.

Example 11

Preparation of4-[5-[1,2-dioxo-3-[4-methyl-benzyloxy]cyclobut-3-en-4-yl]pent-1-yl]-3-[4-methylbenzyloxy]cyclobut-3-ene-1,2-dione

This Example illustrates the preparation of a dimeric squaric acidderivative in which two [4-methylbenzyloxy]cyclobut-3-ene-1,2-dionegroups are linked via a pentamethylene chain.

Triethylamine (423 mg, 4.18 mmole) and p-methylbenzyl bromide (1.47 g,7.96 mmole) were added sequentially to a suspension of4-[5-[1,2-dioxo-3-hydroxycyclobut-3-en-4-yl]pent-1-yl]-3-hydroxy-cyclobut-3-ene-1,2-dione(526 mg, 2.0 mmole, prepared in Example 10 above) in chloroform (15 mL)at room temperature, and the mixture was then heated at reflux for 9hours. The solvent was removed under reduced pressure, and the resultantoil was purified by flash chromatography on silica gel withdichloromethane, followed by ether, as eluents. The product eluted withether, and was obtained as a yellow oil (591 mg, 63% yield). Thestructure of this compound was confirmed by ¹ H and ¹³ C NMRspectroscopy.

EXAMPLES 12-32

Preparation of oxalic acid derivative secondary acid generators

Example 12

Preparation of bis(2-methyl-2-hexyl)oxalate

To a solution of 2-methylhexan-2-ol (4.65 g, 40 mmole) and pyridine(4.74 g, 60 mmole) in tetrahydrofuran (15 mL) was added dropwise at5°-10° C. over a period of 15 minutes a solution of oxalyl chloride(2.54 g, 20 mmole) in THF (6 mL). The resultant suspension was stirredat 20° C. overnight, then diluted with cold water (100 mL) and extractedwith diethyl ether (65 mL). The organic layer was washed with colddilute sulfuric acid, then with aqueous sodium bicarbonate, and finallywith aqueous sodium chloride, then dried over sodium sulfate andevaporated to give the desired product as a pale yellow oil (3.25 g, 62%yield). An analytical sample was obtained by column chromatography onsilica gel with 7% ethyl acetate in hexanes as eluent. The structure ofthis compound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 13

Preparation of bis(α,α-dimethylbenzyl) oxalate

To a solution of α,α-dimethylbenzyl alcohol (5.44 g, 40 mmole) andpyridine (4.74 g) in THF (20 mL) was added dropwise at 5°-10° C. withstirring over a period of 25 minutes a solution of oxalyl chloride (2.54g, 20 mmole) in THF (5 mL). The resultant suspension was stirred at 20°C. for 5 hrs, then poured into 140 mL of 0.5N sulfuric acid kept at 0°C. The oily product which separated was extracted with diethyl ether (60mL) and the ether solution washed with saturated sodium bicarbonate (50mL), and then with saturated aqueous sodium chloride (50 mL). The washedsolution was dried over sodium sulfate and evaporated to give thedesired product as a nearly colorless solid (5.745 g, 88% crude yield).A portion of this product was recrystallized from hexanes to providecolorless needles melting point 76.5°-79° C. The structure of thiscompound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 14

Preparation of bis(p-butoxybenzyl) oxalate

To a solution of p-butoxybenzyl alcohol (1.803 g, 10 mmole) and pyridine(1.185 g, 15 mmole) in dichloromethane (10 mL) was added dropwise over aperiod of 5 minutes a solution of oxalyl chloride (0.635 g, 5 mmole) inmethylene chloride (7 mL) at a temperature of 5°-20° C. The resultantsuspension was stirred at 20° C. overnight, diluted to 50 mL withmethylene chloride, then washed successively with water, dilute sulfuricacid, and aqueous sodium bicarbonate, and finally with brine. The washedsuspension was then dried over sodium sulfate and evaporated to give thedesired product (1.97 g, 76% yield) as colorless plates, melting point113.5°-114.5° C. The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 15

Preparation of bis(α-methylbenzyl)oxalate

To a solution of d,l-α-methylbenzyl alcohol (2.443 g, 20 mmole) andpyridine (2.37 g, 30 mmole) in dichloromethane (20 mL) was added at 5°C. a solution of oxalyl chloride (1.27 g, 10 mmole) in dichloromethane(8 mL). The resultant suspension was stirred at 0° C. for 20 minutes,and then at 20° C. overnight. The suspension was then poured intoice-water and acidified with 1N sulfuric acid (20 mL). The organic layerwas washed with dilute sodium bicarbonate solution, then with brine,dried over sodium sulfate and evaporated to give the desired product asa pale yellow oil (2.661 g, 89% yield). The structure of this compoundwas confirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 16

Preparation of bis(p-methoxy-α-methylbenzyl)oxalate

To a solution of d,l-p-methoxy-α-phenethyl alcohol (3.57 g, 23.4 mmole)in dichloromethane (35 mL) containing 2.78 g (35.8 mmole) of pyridinewas added over a period of 20 minutes at 0° C. a solution of oxalylchloride (1.49 g, 11.8 mmole) in dichloromethane (6 mL). The resultantmixture was stirred at 20° C. for 14 hours, then poured into cold dilutesulfuric acid. The organic layer was washed with cold water, then withdilute sodium bicarbonate, dried over sodium sulfate and evaporated togive the desired product as a colorless oil (4.11 g, 97% yield). A 1.2gram sample of this oil was crystallized from methanol to provide 0.51 gof product as fine matted plates of a mixture of diastereomers meltingat 63°-82° C. The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 17

Preparation of bis(p-methylbenzyl) oxalate

To a solution of p-methylbenzyl alcohol (3.33 g, 27 mmole) in pyridine(7 mL) was added at 0° C. over a period of five minutes oxalyl chloride(0.87 mL, 1.27 g, 10 mmole). The resultant reaction mixture was stirredat 0°-10° C. for one hour, then poured into cold dilute sulfuric acid togive a colorless precipitate, which was collected by filtration andwashed with cold water to give colorless plates. These plates wererecrystallized from methanol and then from hexanes as matted needles.The needles were recrystallized from methanol (30 mL) to provide thedesired product (0.96 g, 32% yield), melting point 100°-100.5° C. Asecond crop of the product (1.20 g, 40% yield) was obtained byconcentration of the mother liquors. The structure of the product wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 18

Preparation of ethyl p-methoxybenzyl oxalate

To a solution of p-methoxybenzyl alcohol (4.49 g, 14.4 mmole) andpyridine (1.92 g, 24.3 mmole) in dichloromethane (10 mL) was added at5°-20° C. a solution of ethyl oxalyl chloride (2.216 g, 16.2 mmole) overa period of 4 minutes. The resultant reaction mixture was stirred at 0°C. for 20 minutes and then at 20° C. overnight. The reaction mixture wasthen poured into ice-water and acidified with 1N sulfuric acid (20 mL).The organic layer was washed with dilute sodium bicarbonate, then withbrine, dried over sodium sulfate and evaporated to give the desiredproduct (3.367 g) as a colorless solid. Recrystallization from hexanesprovided colorless fine irregular prisms, melting point 44°-45° C. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Example 19

Preparation of 2,2-dimethyl-1-[4-methoxybenzyloxalyloxy]prop-3-yl[4-methoxybenzyl]oxalate

A solution of 2,2-dimethylpropane-1,3-diol (24.6 g, 0.236 mole) indichloromethane (200 mL) was added in a slow stream to a solution ofoxalyl chloride (60.0 g, 0.472 mole) in dichloromethane (400 mL) whichhad been pre-cooled to 0° C. using an ice bath, the addition being madeat such a rate that the temperature of the solution did not exceed 10°C. The resultant clear solution was allowed to warm to room temperatureover a period of 30 minutes, and stirred for an additional 30 minutes,then cooled to 0° C. and pyridine (75 g, 0.948 mole) was added, again atsuch a rate as to maintain the temperature of the reaction mixture below10° C. To the resultant yellow suspension was added a solution of4-methoxybenzyl alcohol (65.35 g, 0.473 mole) in dichloromethane (100mL), again keeping the temperature of the reaction mixture to 10° C. orbelow. After the addition had been completed, a cream-coloredprecipitate was observed. The reaction mixture was allowed to warm toroom temperature and stirred overnight.

The mixture was then filtered, and the hygroscopic precipitate ofpyridinium chloride was washed with dichloromethane (2×25 mL). Thecombined organic extracts were washed with: a) water (500 mL) containingconcentrated hydrochloric acid (25 mL); b) water (700 mL) containingsodium hydrogen carbonate (50 g) and c) saturated brine (250 mL). Theorganic layer was then dried over anhydrous sodium sulfate andconcentrated under reduced pressure. The residue was stirred with ether(500 mL) for 10 minutes, then filtered. The precipitate (which was theunwanted by-product, 4-methoxybenzyl oxalate) was washed with more ether(2×25 mL), and the combined ether solutions were concentrated underreduced pressure to give a waxy solid (93.88 g), which resisted attemptsat recrystallization. Purification was, however, effected by triturationwith cold methanol (500 mL) to afford the desired compound (68.5 g, 59%yield) as a white powder, melting point 38°-40° C. The structure of thiscompound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

Example 20

Preparation of 2,2-dimethyl-1-[4-benzyloxy[benzyloxalyloxy]]prop-3-yl[4-methoxybenzyl]oxalate

Example 19 was repeated except that the 4-methoxybenzyl alcohol wasreplaced by 4-benzyloxybenzyl alcohol, to give the above compound in 73%yield. This compound had a melting point of 73°-74 C., and its structurewas confirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 21

Preparation of 1-[4-methoxybenzyloxalyloxy]]hex-6-yl[4-methoxybenzyl]oxalate

Example 19 was repeated except that the 2,2-dimethylpropane-1,3-diol wasreplaced by hexane-1,6-diol, to give the above compound in 49% yield.This compound had a melting point of 114°-115° C., and its structure wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 22

Preparation of cyclohexyl[4-[6-[4-[[cyclohexyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

Part A: Preparation of 4-[1-[4-hydroxymethylphenoxy]hex-6-yloxy]benzylalcohol

4-Hydroxybenzyl alcohol (24.82 g, 0.2 mole) was added to a stirredsuspension of finely ground potassium carbonate (42.0 g, 0.4 mole) indry dimethylformamide (250 mL). The resultant mixture was stirred at 60°C. under dry nitrogen for 10 minutes, then 1,6-dibromohexane (24.4 g,0.1 mole) was added. The reaction mixture was maintained at 60° C. for 5hours, then allowed to cool to room temperature and stirred for 17hours. The reaction mixture was then poured slowly into ice/water (800mL). A tan precipitate formed, which was collected by filtration, washedwith water, and dried in air to give a sticky solid. This material wastriturated with 2-propanol (100 mL) and then with cold water (200 mL),to give the desired product as a powder (13.8 g, 42% yield) which wascollected by (slow and difficult) filtration. The compound melted at96°-110° C., and its structure was confirmed by mass spectroscopy and by¹ H and ¹³ C NMR spectroscopy.

Part B: Preparation of cyclohexyl[4-[6-[4-[[cyclohexyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

A solution of cyclohexanol (2.0 g, 0.02 mole) in dichloromethane (50 mL)was added over a period of 15 minutes to a solution of oxalyl chloride(2.54 g, 0.02 mole) in dichloromethane (50 mL) cooled on an ice bath.The resultant solution was allowed to warm to room temperature over aperiod of 20 minutes, then stirred for a further 30 minutes, then againcooled, using an ice bath, and pyridine (3.16 g, 0.04 mole) was addedover a two minute period. After 5 minutes standing, solid4-[1-[4-hydroxymethylphenoxy]hex-6-yloxy]benzyl alcohol (prepared inPart A above, 3.30 g, 0.01 mole) was added in portions over a period of15 minutes. The slightly turbid solution which formed was allowed towarm to room temperature and stirred for about 30 hours under nitrogen.This solution was then washed with: a) water (100 mL) containingconcentrated hydrochloric acid (10 mL); b) saturated aqueous sodiumhydrogen carbonate (100 mL) and c) saturated brine (50 mL). The organiclayer was then dried over anhydrous sodium sulfate. Charcoal and Celitewere added, and the solution was then filtered through Celite. Afterconcentration of the filtrate under reduced pressure, the residue waspurified by flash chromatography on silica gel with dichloromethane aseluent, giving the desired compound as a pale yellow oil (0.65 g, 10%yield). The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 23

Preparation of adamantyl[4-[6-[4-[[adamantyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

Example 22, Part B was repeated except that the cyclohexanol wasreplaced by an equimolar amount of adamantanol. The above compound wasproduced as a pale yellow oil in 22% yield, and its structure wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 24

Preparation of Menthyl[4-[6-[4-[[menthyloxalyloxy]methyl]phenoxy]hex-6-yloxy]benzyl]oxalate

Example 22, Part B was repeated except that the cyclohexanol wasreplaced by an equimolar amount of d,l-menthol. The above compound wasproduced as a pale yellow oil in 22% yield, and its structure wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

Example 25

Preparation of 2-methacryloxyethyl p-methoxybenzyl oxalate

Part A: Preparation of 2-methacryloxyethyl oxalyl chloride

Oxalyl chloride (50 g) and dichloromethane (50 g) were mixed and cooled,with stirring, in an ice bath to 7°-10° C. To the resultant mixture wasadded 2-hydroxyethyl methacrylate (40 g) over a period of 30 minutes.The resultant mixture was stirred overnight at room temperature under aslow stream of nitrogen, then concentrated on a rotary evaporator forone hour to yield the desired product as a colorless oil (65 g), whichwas sufficiently pure to be used in Part B below without furtherpurification.

Part B: Preparation of 2-methacryloxyethyl p-methoxybenzyl oxalate

p-Methoxybenzyl alcohol (14 g, approximately 0.1 mole) and pyridine (11g, 0.13 mole) were dissolved in dichloromethane (100 mL) and cooled inan ice bath to 2°-4° C. Separately, the product of Part A above (25 g,0.11 mole) was dissolved in dichloromethane (25 mL) and cooled in an icebath. The second solution was added gradually to the first over a periodof 25 minutes while keeping the temperature at 2°-4° C. The resultantreaction mixture was allowed to stand at room temperature overnight,then filtered then filtered through a plug of silica to remove a lowR_(f) impurity detectable by thin layer chromatography (TLC). Thedichloromethane was then removed by evaporation to yield the desiredproduct as a colorless oil (29 g, 91% yield over two stages). TLC withdichloromethane as eluent gave a single spot, R_(f) 0.45. The structureof the product was confirmed by ¹ H NMR spectroscopy indeuterochloroform, the spectrum being as follows: δ=7.28 (doublet, 2H);6.83 (doublet, 2H); 6.05 (singlet, 1H); 5.50 (singlet, 1H); 5.17(singlet, 2H); 4.43 (triplet, 2H); 4.37 (triplet, 2H); 3.72 (singlet,3H); and 1.86 (singlet, 3H) ppm.

Example 26

Preparation of poly(2-methacryloxyethyl p-methoxybenzyl oxalate)

The product of Example 25 above (29 g) was dissolved in toluene (200 mL)and azobis(isobutyronitrile) (AIBN; 0.3 g) was added. The resultantmixture was heated at 65° C. under nitrogen for 16 hours, additionalAIBN (0.2 g) was added, and the mixture was heated under nitrogen for afurther 24 hours. A polymeric product precipitated as a swollen gel,from which the supernatant liquor was decanted. The gel was washedrepeatedly with diethyl ether, whereupon it deswelled and hardened. Thewashed polymer was dried in vacuo at 40° C. to yield the desired polymer(26 g, approximately 90% yield) as a non-sticky white solid, glasstransition temperature (T_(g)) 65° C., decomposing at 210° C. in theabsence of any catalyst.

Example 27

Preparation of 4-methacryloxybutyl p-methoxybenzyl oxalate

Example 25 above was repeated, except that 4-hydroxybutyl methacrylatewas substituted for 2-hydroxyethyl methacrylate. The product wasobtained as a colorless oil (yield 85%) and its structure was confirmedby ¹ H NMR spectroscopy in deuterochloroform, the spectrum being asfollows: δ=7.28 (doublet, 2H); 6.83 (doublet, 2H); 6.05 (singlet, 1H);5.50 (singlet, 1H); 5.17 (singlet, 2H); 4.23 (triplet, 2H); 4.13(triplet, 2H); 3.72 (singlet, 3H); 1.86 (singlet, 3H) and 1.72(multiplet, 4H) ppm.

Example 28

Preparation of poly(4-methacryloxybutyl p-methoxybenzyl oxalate)

The product of Example 27 above (5 g) was dissolved in toluene (25 mL)and AIBN (0.025 g) was added. The resultant mixture was heated at 65° C.under nitrogen for 16 hours, and then poured into hexane, whereupon thedesired polymeric product precipitated, T_(g) approximately 50° C.,decomposing above 200° C. in the absence of any catalyst.

Example 29

Preparation of 4-benzyloxybenzyl 2-methacryloxyethyl oxalate

Example 25, Part B above was repeated, except that 4-benzyloxybenzylalcohol was substituted for p-methoxybenzyl alcohol. The product wasobtained as a white solid, melting point 4°-42° C. (yield 85%) and itsstructure was confirmed by ¹ H NMR spectroscopy in deuterochloroform,the spectrum being as follows: δ=7.4 (multiplet, 5H); 7.28 (doublet,2H); 6.85 (doublet, 2H); 6.07 (singlet, 1H); 5.52 (singlet, 1H); 5.23(singlet, 2H); 5.02 (singlet, 2H); 4.45 (triplet, 2H); 4.35 (triplet,2H); and 1.88 (singlet, 3H) ppm.

This monomer was converted to its homopolymer in the same manner asdescribed in Example 28 above.

Example 30

Preparation of ethyl 4-(4-vinylbenzyloxy)benzyl oxalate

Part A: Preparation of 4-(4-vinylbenzyloxy)benzyl alcohol

A solution of potassium hydroxide pellets (3.2 g, 0.05 mole) in 50 mL ofethanol was prepared and stirred in a flask under nitrogen. Separately,p-hydroxybenzyl alcohol (6.2 g, 0.05 mole) and p-vinylbenzyl chloride(7.6 g, 0.05 mole) were dissolved in 50 ml of ethanol. The secondsolution was added to the first with stirring under nitrogen, and theresultant mixture was heated to 65° C. overnight. The reaction mixturewas then cooled to room temperature and filtered, and solvent wasremoved from the filtrate on a rotary evaporator to give a tan solid.This solid was extracted with warm water, filtered off and dried,extracted with petroleum ether, filtered off and finally recrystallizedfrom toluene/hexane to yield the desired product as a colorless solid (6g, approximately 50% yield), melting point 110°-112° C. Its structurewas confirmed by ¹ H NMR spectroscopy in deuterochloroform, the spectrumbeing as follows: δ=7.38 (two doublets, J=10 Hz, 4H); 7.23 (doublet, J=10 Hz, 2H); 6.85 (doublet, J=10 Hz, 2H); 6.67 (two doublets, J=10 and 18Hz, 1H); 5.72 (doublet, J=18 Hz, 1H); 5.21 (doublet, J=10 Hz, 1H); 5.0(singlet, 2H); 4.57 (singlet, 2H); and 1.6 (singlet, 1H) ppm.

Part B: Preparation of ethyl 4-(4-vinylbenzyloxy)benzyl oxalate

The product of Part A above (4.8 g, 0.02 mole) and pyridine (2.0 g,0.025 mole) were dissolved in dichloromethane (50 mL) and cooled to10°-12° C. To this solution was added over a period of 10 minutes asolution of ethyloxalyl chloride (3 g, 0.022 mole) in dichloromethane (5mL). TLC of the reaction mixture after the addition had been completedindicated that only a trace of the alcohol starting material remained.The reaction mixture was then filtered through a plug of silica toremove the pyridine salt produced, and the filtrate was concentrated toproduce the desired produce as white crystals (approximately 90% yield)melting point 93° C. Its structure was confirmed by ¹ H NMR spectroscopyin deuterochloroform, the spectrum being as follows: δ=7.35 (twodoublets, J=10 Hz, 4H); 7.25 (doublet, J=10 Hz, 2H); 6.85 (doublet, J=10Hz, 2H); 6.67 (two doublets, J=10 and 18 Hz, 1H); 5.72 (doublet, J=18Hz, 1H); 5.21 (doublet, J=10 Hz, 1H); 5.18 (singlet, 2H); 5.0 (singlet,2H); 4.27 (quadruplet, J=8 Hz, 2H); and 1.28 (triplet, J=8 Hz, 3H) ppm

Example 31

Preparation of poly(ethyl 4-(4-vinylbenzyloxy)benzyl oxalate)

The product of Example 30 above (approximately 2 g) was dissolved intoluene (25 mL) and AIBN (0.01 g) was added. The resultant mixture washeated at 65° C. under nitrogen for 16 hours. Proton NMR analysisindicated only about 50% polymerization, so additional AIBN (0.015 g)was added, and the mixture was heated at 65° C. under nitrogen for afurther 16 hours. The resultant slightly viscous solution was pouredinto a 1:1 v/v mixture of diethyl ether and petroleum ether toprecipitate the polymer, which was then treated with petroleum ether fordeswelling. After drying, the desired polymer (approximately 0.7 g) wasobtained as an off-white powder. Proton NMR analysis revealed no traceof remaining monomer.

Example 32

Preparation of 4-(4-vinylbenzyloxy)benzyl oxalate)

3-Phenylpropyloxalyl chloride was prepared by reacting oxalyl chloridewith 3-phenylpropanol in dichloromethane at 10° C. Example 30, Part Bwas then repeated using the 3-phenylpropyloxalyl chloride in place ofethyloxalyl chloride, to produce the product as fine white crystals,melting point 80° C. (81% yield). Its structure was confirmed by ¹ H NMRspectroscopy in deuterochloroform, the spectrum being as follows:δ=7.1-7.4 (multiplet, 9H); 7.27 (doublet, 2H); 6.87 (doublet, 2H); 6.67(two doublets, 1H); 5.72 (doublet, 1H); 5.22 (doublet, 1H); 5.20(singlet, 2H); 5.03 (singlet, 2H); 4.21 (triplet, 2H); 2.65 (triplet,2H); and 2.0 (two triplets, 2H) ppm.

Polymerization of this monomer in the same manner as in Example 31 abovegave the corresponding polymer in a yield of 75%. This polymer as foundto give good results as a secondary acid generator.

Imaging and other processes of the invention

EXAMPLE 33

Polymerization process of the invention using squaric acid derivative

This Example illustrates a process of the present invention in which thesuperacid generated after infra-red and ultra-violet irradiation is usedto cause polymerization of a difunctional epoxy monomer.

A coating fluid was prepared by dissolving a silicone diepoxy monomer ofthe formula: ##STR9## (supplied by General Electric Company, 40 mg),t-butyl-anthracene (5 mg; a precursor sensitizer), an infra-red dye ofthe formula: ##STR10## (see U.S. Pat. No. 4,508,811, 0.3 mg),(4-octyloxyphenyl)phenyliodonium hexafluoroantimonate (8 mg, prepared asper U.S. Pat. No. 4,992,571), and poly(vinyl chloride) (supplied byAldrich Chemical Company, Milwaukee, Wis., 30 mg) in methyl ethyl ketone(MEK, 0.6 mL). This solution was coated on to a poly(ethyleneterephthalate) base 4 mil(101 μm) in thickness (ICI Type 3295, suppliedby ICI Americas, Inc., Wilmington, Del.) using a number 18 coating rod.

The coated side of the resultant coating was exposed to infra-redradiation from a GaAlAs semiconductor diode laser emitting at 822 nm,which delivered 125 mW to the medium. The laser output was focussed to aspot approximately 33×3 μm. The medium was wrapped around a drum whoseaxis was perpendicular to the incident laser beam. Rotation of the drumabout its axis and simultaneous translation in the direction of the axiscaused the laser spot to write a helical pattern on the medium. Thepitch of the helix was 33 μm, chosen so that none of the medium was leftunexposed between adjacent turns. In this arrangement, the exposurereceived by the medium was inversely proportional to the speed ofrotation of the drum (here measured as a linear speed at the mediumsurface). Separate bands of the medium were exposed at 2.0, 2.5, 3.0,3.5 and 4.0 m/s.

Following this infra-red exposure, the entire coating was exposed for 70seconds to ultra-violet radiation from a Universal UV unit (nominallyemitting at 375 nm) supplied by Gelman Instrument Company. The coatingwas next heated on a hotplate at 100° C. for 20 seconds, after which thecoating was developed by washing sequentially with methyl ethyl ketoneand dichloromethane. Residual material was finally removed by sonicationin a bath of methyl ethyl ketone for three minutes. In all areas whichhad received the infra-red exposure, insoluble polymeric materialremained adhering to the polyester base and was not removed by thesolvent treatment or sonication, whereas in all other areas of the film,including those areas which had received ultra-violet but not infra-redirradiation, no polymeric material was left adhering to the base afterthese treatments.

EXAMPLE 34

Imaging process of the invention using a squaric acid derivative as acidgenerator

This Example illustrates an imaging process of the invention in whichthe imaging medium contains a secondary acid generator which amplifiesthe unbuffered superacid present in infra-red exposed areas followingthe infra-red and ultra-violet irradiations.

Two coating fluids were prepared as follows:

Fluid A: t-Butyl-anthracene (7 mg), the infra-red dye IR1 described inExample 33 above (0.3 mg), (4-octyloxyphenyl)phenyliodoniumhexafluoroantimonate (5 mg),3,4-bis(4-methylbenzyloxy)cyclobut-3-en-1,2-dione (20 mg) and acopolymer of vinylidene chloride and acrylonitrile (Saran Resin F120,available from Aldrich Chemical Company, Milwaukee, Wis., 30 mg) weredissolved in methyl ethyl ketone (0.6 mL).

Fluid B: A leuco dye of the formula: ##STR11## (15 mg; this leuco dyemay prepared by the procedure described in U.S. Pat. No. 4,345,017) anda hindered amine base (HALS-62, available from Fairmount ChemicalCompany, Inc., 117 Blanchard Street, Newark N.J. 07105, 7 mg) weredissolved in 1:1 MEK:chloroform. Saran Resin F120 (available fromAldrich Chemical Company, Milwaukee, Wis., 30 mg) dissolved in methylethyl ketone (0.3 mL) was added to the resultant solution.

These coating fluids were separately coated on to poly(ethyleneterephthalate) base of 4 mil (101 μm) in thickness (ICI Type 3295,supplied by ICI Americas, Inc., Wilmington Del.) using a number 18coating rod to form Films A and B respectively.

Film A was exposed through the polyester base to infra-red radiationfrom a GaAlAs semiconductor diode laser in the same way as in Example 33above. Following the infra-red exposure, the entire coated side of FilmA was exposed to the unfiltered output of a low pressure mercury UVlamp, model B-100 (supplied by Black Light Eastern, a division ofSpectronics Corporation, Westbury, Long Island, N.Y.) for 47 seconds.Film A was next heated on a hotplate at 117° C. for 15 seconds, afterwhich it was laminated at 240° F. (116° C.) and 60 psi (0.4 MPa) to FilmB, with the two coated sides in contact. Table 2 below shows the greenoptical densities achieved for various infra-red exposures; thesedensities were measured using an X-Rite 310 photographic densitometer,supplied by X-Rite, Inc., Grandville, Mich., with the appropriatefilter.

                  TABLE 2                                                         ______________________________________                                        Scanning speed (m/sec)                                                                         Green optical density                                        ______________________________________                                        No IR exposure   0.07                                                         2.0              2.68                                                         2.5              2.81                                                         3.0              2.73                                                         3.5              2.69                                                         4.0              2.95                                                         ______________________________________                                    

From the data in Table 2, it will be seen that the green optical densityachieved in the imaged areas was independent of the scanning speedwithin the range shown in Table 2. Further experiments indicated that athigher scanning speeds, very little magenta dye density developed,presumably because so little superacid was generated during theinfra-red irradiation that, even after the ultra-violet irradiation, thequantity of superacid generated did not exceed the threshold required toprotonate all the infra-red dye and hence leave unbuffered superacidpresent in the infra-red exposed areas. Accordingly, at these highscanning speeds, even in the infra-red exposed areas, there was nounbuffered superacid available to catalyze the decomposition of the acidgenerator, so little or no production of the second acid took place andlittle magenta color developed.

Further experiments also indicated that if the ultra-violet exposure wasless than 40 or more than 55 seconds, no significant difference inoptical density was seen between the areas which had received infra-redirradiation and those which had not; if the ultra-violet irradiation wastoo short, little or no dye density developed in any part of the film,while if the ultra-violet irradiation was too long, the whole filmdeveloped the maximum dye density. Presumably, if the ultra-violetirradiation is too short and thus too little superacid precursor isdecomposed during this irradiation, even in infra-red exposed areas thequantity of superacid present following the ultra-violet irradiationwill not exceed the aforementioned threshold, no unbuffered superacidwill be present in the infra-red exposed areas during the heating step,no acid amplification will occur, and little or no magenta dye densitywill result. On the other hand, if the ultra-violet irradiation is toolong and thus too much superacid precursor is decomposed during thisirradiation, even in areas not exposed to infrared radiation thequantity of superacid present following the ultra-violet irradiationwill exceed the aforementioned threshold, unbuffered superacid will bepresent throughout the film and acid amplification and dye color changewill occur in all areas.

EXAMPLE 35

Imaging process of the invention using a single sheet medium

This Example illustrates an imaging process of the invention generallysimilar to that in Example 34 above, but in which the imaging mediumcomprises a single sheet rather than two sheets which are laminatedtogether following the ultra-violet irradiation.

Two dispersions were prepared as follows:

Dispersion A:

De-ionized water (60 mL) was added dropwise to a magnetically stirredsolution of a surfactant (Aerosol TR-70, adjusted with potassiumhydroxide to pH 6, 0.65 g), the leuco dye used in Example 34 above (2.5g), a base (HALS-62, supplied by Fairmount Chemical Company, 0.25 g) anda polymeric binder (Elvacite 2043, supplied by DuPont de Nemours,Wilmington, Del., 2.75 g) in dichloromethane (46 mL). The resultant,very viscous mixture was sonicated, causing the viscosity to decrease,and then the mixture was allowed to stir overnight at room temperature,during which time the dichloromethane evaporated. A fluorinatedsurfactant (FC-120, supplied by Minnesota Mining and ManufacturingCorporation, St. Paul, Minn., 56 mg of a 25% aqueous solution) was thenadded.

Dispersion B:

De-ionized water (53.5 mL) was added dropwise to a magnetically stirredsolution of a surfactant (Aerosol TR-70, adjusted with potassiumhydroxide to pH 6, 0.58 g), a base (HALS-63, supplied by FairmountChemical Company, 2.45 g) and a polymeric binder (Elvacite 2043,supplied by DuPont de Nemours, 2.45 g) in dichloromethane (53.5 g). Theresultant, very viscous mixture was sonicated, causing the viscosity todecrease, and then the mixture was allowed to stir overnight at roomtemperature, during which time the dichloromethane evaporated.

2 mL of Dispersion A was then combined with 1 mL of Dispersion B andpoly(vinyl alcohol) (Vinol 540, supplied by Air Products Corporation,Allentown, Penn., 1 mL of 5% aqueous solution). The resultant coatingfluid was then overcoated, using a number 8 coating rod, on to Film Aprepared in Example 34 above.

The imaging medium thus prepared, which comprised a single sheet havingboth an acid-generating layer and a color-forming layer, was exposedthrough the polyester base to infra-red radiation from a GaAlAs laser inthe same manner as in Example 33 above. Following this infra-redirradiation, the entire coating was exposed for 200 seconds, through thepolyester base, to ultraviolet radiation from the aforementioned lowpressure mercury UV lamp, model B-100 equipped with a 365 nminterference filter (supplied by Corion Corporation, Holliston, Mass.).The power measured at the film plane in the arrangement used was 0.3mW/cm². The coating was then heated on a hotplate at 115° C. for 60seconds. Table 3 below shows the green optical density achieved forvarious infra-red exposures, measured in the same manner as in Example34 above.

                  TABLE 3                                                         ______________________________________                                        Scanning speed (m/sec)                                                                         Green optical density                                        ______________________________________                                        No IR exposure   0.03                                                         2.0              1.04                                                         2.5              1.34                                                         3.0              1.35                                                         3.5              0.97                                                         4.0              0.41                                                         ______________________________________                                    

From the data in Table 3 , it will be seen that the optical densityachieved was independent of scanning speed only for scanning speedsbelow 3.5 m/s. Presumably, at higher scanning speeds, too littlesuperacid precursor is decomposed during the infra-red irradiation, sothat even in infra-red exposed areas the quantity of superacid presentfollowing the ultra-violet irradiation will not exceed theaforementioned threshold, little or no unbuffered superacid will bepresent in the infra-red exposed areas during the heating step, littleor no acid amplification will occur, and a reduced magenta dye densitywill result.

EXAMPLE 36

Imaging process of the invention using an oxalic acid derivative as acidgenerator

This Example illustrates an imaging process of the invention in whichthe imaging medium contains an oxalic acid derivative secondary acidgenerator which amplifies the unbuffered superacid present in infra-redexposed areas following the infra-red and ultra-violet irradiations.

Two coating fluids were prepared as follows:

Fluid A 1-Vinylpyrene (20 mg), the infra-red dye IR1 described inExample 33 above (3 mg), (4-n-octyloxyphenyl)phenyliodoniumhexafluoroantimonate (25 mg),2,2-dimethyl-1-[4-methoxybenzyloxalyloxy]prop-3-yl-[4-methoxybenzyl]oxalate(100 mg, prepared as described in Example 19 above), fluorinatedsurfactant FC-431 (available from Minnesota Mining and ManufacturingCo., St. Paul, Minn., 50 mg of a 2% solution in 2-butanone) and 3.5 g ofa 5% w/w solution of polystyrene of molecular weight approximately45,000 (available from Aldrich Chemical Company, Milwaukee, Wis.) in2-butanone were combined.

Fluid B De-ionized water (40 mL) was added dropwise to a magneticallystirred solution of a surfactant (Aerosol TR-70, supplied by AmericanCyanamid Co., Wayne, N.J. 07470, adjusted with potassium hydroxide to pH6, 0.34 g), indicator dye3,3-bis-[1-butyl-2-methyl-1H-indol-3-yl]-1-isobenzofuranone (soldcommercially under the tradename Copikem 20 by Hilton Davis Co., 2235Langdon Farm Road, Cincinnati, Ohio 45237, 2.0 g), a hindered amine base(Tinuvin 292, available from Ciba-Geigy Co., Ardsdale, N.Y., 0.25 g) anda polymeric binder (Elvacite 2043, available from E. I. DuPont deNemours, Wilmington, Del., 2.5 g) in dichloromethane (40 mL). Theresultant mixture was sonicated, and became very viscous; furthersonication caused its viscosity to decrease (more deionized water wasadded during sonication), and the sonicated mixture was then passedthrough a microfluidizer. Residual dichloromethane was removed underreduced pressure to give a final dispersion 5.3% in solids. Thisdispersion below was diluted with 20% of its own weight of a 7.3 %solution of poly(vinyl alcohol) (Vinol 540, supplied by Air ProductsCorporation, Allentown, Penn.; this material was dialyzed before use) inwater.

An imaging medium was prepared by coating Fluid A on to a reflectivebase of 6 mil (152 μm) thickness (ICI Melinex, available from ICIAmericas, Inc., Wilmington, Del.) using a #6 coating rod, followed byFluid B, which was coated onto the dried coating of Fluid A with a #5coating rod.

The imaging medium thus prepared was imagewise exposed to infra-redradiation form a GaAlAs semiconductor diode laser in the same way as inExample 33 above. Following the infra-red exposure, the entire film wasexposed to ultraviolet radiation from a 1000 W mercury vapor lamp(filtered to remove wavelengths below about 330 nm) in a nuArc 26-1K UVexposure system (available from nuArc company, Inc., 6200 W. Howard St.,Niles, Ill. 60648). The irradiance at the film plane was 16 mJ/cm²,measured using a "Light Bug" radiometer, type IL390B, available fromInternational Light, Inc., Newburyport, Mass. 01950. Regions of the filmwhich had been exposed at 2.0, 2.5, 3.0, 3.5, 4.0 and 5.0 meters/secondexhibited the same green optical density, as measured using an X-Rite310 photographic densitometer, supplied by X-Rite, Inc., Grandville,Mich., with the appropriate filter. This density, designated D_(max),was 1.10. Regions of the film which had not been exposed to infra-redradiation exhibited a green density (D_(min)) of 0.18.

From the foregoing, it will be seen that the present invention providesa process for generation of a superacid (and optionally a strong secondacid) using radiation of wavelengths (preferably near infra-redwavelengths) to which conventional superacid precursors are notsensitive. The superacid or second acid thus generated can be used tocarry out any acid-dependent reaction for which superacids and otheracids have hitherto been used. In particular, preferred embodiments ofthe invention permit the production of high resolution images usinginfra-red lasers, with high sensitivity of the imaging medium.

We claim:
 1. An imaging medium comprising:a superacid precursor; and adye capable of absorbing actinic radiation of a first wavelength, thesuperacid precursor being decomposed to form a superacid by actinicradiation of a second wavelength shorter than the first wavelength inthe absence of the dye, but not being decomposed by actinic radiation ofthe first wavelength in the absence of the dye, the superacid precursorbeing decomposed by actinic radiation of the first wavelength in thepresence of the dye, the superacid produced by decomposition of thesuperacid precursor in the presence of the dye being capable of forminga protonated product derived from the dye; and a secondary acidgenerator capable of being thermally decomposed to form a secondaryacid, the thermal decomposition of the secondary acid generator beingcatalyzed in the presence of the superacid.
 2. An imaging mediumaccording to claim 1 further comprising a polymeric binder in which thesuperacid precursor and the dye are dispersed.
 3. An imaging mediumaccording to claim 1 wherein the dye is a squarylium dye.
 4. An imagingmedium according to claim 1 wherein the superacid precursor comprises aniodonium compound.
 5. An imaging medium according to claim 1 wherein thesecondary acid generator is a 3,4-disubstituted-cyclobut-3-ene-1,2-dionein which at least one of the 3- and 4-substituents consists of an oxygenatom bonded to the squaric acid ring, and an alkyl or alkylene group, apartially hydrogenated aryl or arylene group, or an aralkyl group bondedto said oxygen atom, said 3,4-disubstituted-cyclobut-3-ene-1,2-dionebeing capable of decomposing so as to cause replacement of the or eachoriginal alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkyloxy group ofthe derivative with a hydroxyl group.
 6. An imaging medium according toclaim 5 wherein the 3,4-disubstituted-cyclobut-3-ene-1,2-dione isselected from the group consisting of:(a) primary and secondary estersof squaric acid in which the α-carbon atom bears a non-basiccation-stabilizing group; (b) tertiary esters of squaric acid in whichthe α-carbon atom does not have an sp² or sp hybridized carbon atomdirectly bonded thereto; and (c) tertiary esters of squaric acid inwhich the α-carbon atom does have an sp² or sp hybridized carbon atomdirectly bonded thereto, provided that this sp² or sp hybridized carbonatom, or at least one of these sp² or sp hybridized carbon atoms, ifmore than one such atom is bonded directly to the α-carbon atom, isconjugated with an electron-withdrawing group.
 7. An imaging mediumaccording to claim 5 wherein the3,4-disubstituted-cyclobut-3-ene-1,2-dione is of one of the followingformulae: ##STR12## in which R^(l) is an alkyl group, a partiallyhydrogenated aromatic group, or an aralkyl group, and R² is a hydrogenatom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino,alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino,dialkylphosphoxy or trialkylsilyl group, subject to the proviso thateither or both of the groups R¹ and R² may be attached to a polymer;##STR13## in which R¹ and R³ independently are each an alkyl group, apartially hydrogenated aryl group or an aralkyl group, subject to theproviso that either or both of the groups R¹ and R³ may be attached to apolymer; and ##STR14## in which n is 0 or 1, and R⁴ is an alkylene groupor a partially hydrogenated arylene group; or the squaric acidderivative comprises at least one unit of the formula: ##STR15## inwhich n is 0 or 1, and R⁵ is an alkylene or partially hydrogenatedarylene group.
 8. An imaging medium according to claim 1 wherein thesecondary acid generator is an oxalic acid diester and the secondaryacid generated therefrom is oxalic acid or an oxalic acid monoesterhaving one carboxyl group.
 9. An imaging medium according to claim 8wherein the oxalic acid diester is selected from the group consistingof:(a) primary and secondary esters of oxalic acid in which the α-carbonatom bears a non-basic cation-stabilizing group; (b) tertiary esters ofoxalic acid in which the α-carbon atom does not have an sp² or sphybridized carbon atom directly bonded thereto; (c) tertiary esters ofoxalic acid in which the α-carbon atom does have an sp² or sp hybridizedcarbon atom directly bonded thereto, provided that this sp² or sphybridized carbon atom, or at least one of these sp² or sp hybridizedcarbon atoms, if more than one such atom is bonded directly to theα-carbon atom, is conjugated with an electron-withdrawing group; (d) anester formed by condensation of two moles of an alcohol with thebis(hemioxalate) of a diol, provided that the ester contains at leastone ester grouping of type (a), (b) or (c); (e) polymeric oxalatesderived from polymerization of oxalate esters having an ethylenicallyunsaturated group, provided that the ester contains at least one estergrouping of type (a), (b) or (c); and (f) condensation polymers ofoxalates, provided that the ester contains at least one ester groupingof type (a), (b) or (c) above.
 10. An imaging medium according to claim8 wherein the oxalic acid diester is one which begins to decomposethermally at a temperature in the range of about 140° to about 180° C.,as measured by differential scanning calorimetry in a nitrogenatmosphere at a 10° C./minute temperature ramp, in the absence of anycatalyst.
 11. An imaging medium according to claim 1 further comprisingan acid-sensitive material capable of changing color in the presence ofthe secondary acid.
 12. An imaging medium according to claim 11 whereinthe acid-sensitive material is in admixture with an amount of a basicmaterial insufficient to neutralize all the secondary acid capable ofbeing liberated by heating of the secondary acid generator.
 13. Animaging medium according to claim 12 wherein the acid-sensitive materialis present in a layer or phase separate from the layer or phasecontaining the superacid precursor and the secondary acid generator. 14.An imaging medium comprising:a superacid precursor and an infra-red dyecapable of absorbing infra-red radiation having a wavelength within therange of about 700 to about 1200 nm, the superacid precursor beingcapable of being decomposed by ultraviolet radiation having a wavelengthin the range of about 180 to about 400 nm to form a superacid, thesuperacid precursor not being decomposed by infra-red radiation having awavelength within the range of about 700 to about 1200 nm in the absenceof the infra-red dye.
 15. An imaging medium according to claim 14further comprising at least one of:(a) a secondary acid generatorcapable of acid-catalyzed decomposition by the unbuffered superacid toform a secondary acid; (b) a monomer or oligomer which polymerizes inthe presence of the unbuffered superacid; (c) a polymer whichdepolymerizes in the presence of the unbuffered superacid; (d) a polymerthe solubility of which in a solvent changes in the presence of theunbuffered superacid; and (e) a polymer the adhesion of which to amaterial changes in the presence of the unbuffered superacid.
 16. Animaging medium according to claim 14 further comprising a polymericbinder in which the superacid precursor and the infra-red dye aredispersed.